Redesign function configuration in bindgen!

[alexcrichton]

This commit is a redesign of how function-level configuration works in
Wasmtime's bindgen! macro. The main goal of this redesign is to
better support WASIp3 and component model async functions. Prior to this
redesign there was a mish mash of mechanisms to configure behavior of
imports/exports:

• The async configuration could turn everything async, nothing async,
  only some imports async, or everything except some imports async.

• The concurrent_{imports,exports} keys were required to explicitly
  opt-in to component model async signatures and applied to all
  imports/exports.

• The trappable_imports configuration would indicate a list of imports
  allowed to trap and it had special configuration for everything,
  nothing, and only a certain list.

• The tracing and verbose_tracing keys could be applied to either
  nothing or all functions.

Overall the previous state of configuration in bindgen! was clearly a
hodgepodge of systems that organically grew over time. In my personal
opinion it was in dire need of a refresh to take into account how
component-model-async ended up being implemented as well as
consolidating the one-off systems amongst all of these configuration
keys. A major motivation of this redesign, for example, was to inherit
behavior from WIT files by default. An async function in WIT should
not require concurrent_* keys to be configured, but rather it should
generate correct bindings by default.

In this commit, all of the above keys were removed. All keys have been
replaced with imports and exports configuration keys. Each behaves
the same way and looks like so:

bindgen!({
    // ...
    imports: {
        // enable tracing for just this function
        "my:local/interface/func": tracing,

        // enable verbose tracing for just this function
        "my:local/interface/other-func": tracing | verbose_tracing,

        // this is blocking in WIT, but generate async bindings for
        // it
        "my:local/interface/[method]io.block": async,

        // like above, but use "concurrent" bindings which have
        // access to the store.
        "my:local/interface/[method]io.block-again": async | store,

        // everything else is, by default, trappable
        default: trappable,
    },
});

Effectively all the function-level configuration items are now bitflags.
These bitflags are by default inherited from the WIT files itself (e.g.
async functions are async | store by default). Further configuration
is then layered on top at the desires of the embedder. Supported keys are:

• async - this means that a Rust-level async function should be
  generated. This is either CallStyle::Async or
  CallStyle::Concurrent as it was prior, depending on ...

• store - this means that the generated function will have access to
  the store on the host. This is only implemented right now for async | store functions which map to CallStyle::Concurrent. In the future
  I'd like to support just-store functions which means that you could
  define a synchronous function with access to the store in addition to
  an asynchronous function.

• trappable - this means that the function returns a
  wasmtime::Result<TheWitBindingType>. If trappable_errors is
  applicable then it means just a Result<TheWitOkType, TrappableErrorType> is returned (like before)

• tracing - this enables tracing! integration for this function.

• verbose_tracing - this logs all argument values for this function
  (including lists).

• ignore_wit - this ignores the WIT-level defaults of the function
  (e.g. ignoring WIT async).

The way this then works is all modeled is that for any WIT function
being generated there are a set of flags associated with that function.
To calculate the flags the algorithm looks like:

1. Find the first matching rule in the imports or exports map
   depending on if the function is imported or exported. If there is no
   matching rule then use the default rule if present. This is the
   initial set of flags for the function (or empty if nothing was
   found).

2. If ignore_wit is present, return the flags from step 1. Otherwise
   add in async | store if the function is async in WIT.

The resulting set of flags are then used to control how everything is
generated. For example the same split traits of today are still
generated and it's controlled based on the flags. Note though that the
previous HostConcurrent trait was renamed to HostWithStore to make
space for synchronous functions in this trait in the future too.

The end result of all these changes is that configuring imports/exports
now uses the exact same selection system as the with replacement map,
meaning there's only one system of selecting functions instead of 3.
WIT-level async is now respected by default meaning that bindings work
by default without further need to configure anything (unless more
functionality is desired).

One final minor change made here as well is that auto-generated
instantiate methods are now always synchronous and an
instantiate_async method is unconditionally generated for async mode.
This means that bindings always generate both functions and it's up to
the embedder to choose the appropriate one.

Closes #11246
Closes #11247

[alexcrichton]

One other large-ish change now here, *WithStore is always generated no matter what. That'll make the generated docs slightly more confusing but I think it's worth the tradeoff of enabling bindings to always assume the trait is there.

[pchickey]

Thanks, this is a nice improvement! In the future I'd probably like it to be tracing or tracing(verbose) instead of another keyword for verbose_tracing, but its not necessary to do that now.