Rust Engine: print infrastructure

[W95Psp]

This PR adds a improves the printing and backend infrastructure.

The PrettyAst trait

The main change of this PR in the introduction of the PrettyAst trait, that reduces boilerplate.
Instead of implementing pretty::Pretty for every type of the AST, one must only implement PrettyAst, which has default methods.
The default methods print debugging information that makes it easy to iterate when writting a printer.

Helper macros

The pretty library we are using for pretty printing uses an explicit allocator: whenever we must render a node, the allocator needs to be specified expclitely.
As it is visible in the existing lean printer, this is very verbose.

This PR introduces pretty_ast::install_pretty_helpers!, a macro that hides this allocator away. This macro basically creates partial applications of the allocator method using a local variable.

Instead of doing:

docs![allocator, "something", intersperse!(docs![allocator, node], docs![allocator, ",", allocator.space()])]

You can do:

install_pretty_helpers!(allocator);
docs!["something", intersperse!(docs![node], docs![",", space!()])]

We also introduce the macro prepend_associated_functions_with!(something) that prepends something to each body of associated function.

Combining the trait PrettyAst, the macro prepend_associated_functions_with and the macros given by install_pretty_helpers, we get implicit allocator across entire printers.
See backends::rust for an example.

Rust Printer

This PR introduces a dummy placeholder Rust printer.

[clementblaudeau]

This PR addresses a real issue of verbosity when writing backends. It will make the code more concise and provide better error messages when parts of the ast are unsupported.

[jschneider-bensch]

Clément already brought up a lot of things I wanted to ask about, and you already addressed those.
I have, as usual, some questions, but I think the code is good!

[W95Psp]

Thanks @clementblaudeau and @jschneider-bensch for your reviews! :)
Let's merge