Refactor instantiation to be more async-friendly

Instantiation right now uses a recursive `instantiate` function since it
was relatively easy to write that way, but this is unfortunately not
factored in a way friendly to the async implementation in #2434. This
commit refactors the function to instead use an iterative loop and
refactors code in such a way that it should be easy to rebase #2434 on
top of this change. The main goal is to make the body of `Instance::new`
as small as possible since it needs to be duplicated with
`Instance::new_async`.

Author: alexcrichton