Add initial support for WebAssembly Interface Types

[alexcrichton]

This commit adds initial support for WebAssembly Interface
Types to wasmtime. This is all intended to be quite
experimental, so experimental in fact that even the name of the
proposal is still in flux. (this has otherwise been known as "host
bindings" or "webidl bindings" or "wasm bindings").

The goal of this commit is to start adding support the wasmtime set of
crates for WebAssembly Interface Types. A new wasmtime-interface-types
crate has been added with very basic support for dynamically invoking
and inspecting the various bindings of a module. This is in turn powered
by the wasm-webidl-bindings crate which is shared with the
wasm-bindgen CLI tool as a producer of this section.

Currently the only integration in wasmtime-the-binary itself is that
when passed the --invoke argument the CLI will now attempt to invoke
the target function with arguments as parsed from the command line
itself. For example if you export a function like:

fn render(&str) -> String

Then passing --invoke render will require one argument on the command
line, which is the first argument as a string, and the return value is
printed to the console. This differs from today's interpretation of
--invoke where it is a failure if the invoked function takes more than
one argument and the return values are currently ignored.

This is intended to also be the basis of embedding wasmtime in other
contexts which also want to consume WebAssembly interface types. A
Python extension is also added to this repository which implements the
wasmtime package on PyPI. This Python extension is intended to make it
as easy as pip3 install wasmtime to load a WebAssembly file with
WebAssembly Interface Types into Python. Extensions for other languages
is of course possible as well!

One of the major missing pieces from this is handling imported functions
with interface bindings. Currently the embedding support doesn't have
much ability to support handling imports ergonomically, so it's intended
that this will be included in a follow-up patch.

Co-authored-by: Yury Delendik ydelendik@mozilla.com

[alexcrichton]

Some more logistical notes on this are:

• There are three new builders of the Python extension all present on CI (Windows/Mac/Linux). The main thing to note here is that a nightly version of Rust is used (pinned for now to hopefully reduce breakage)
• The Python extension is published to GitHub artifacts for releases in the same manner that the wasmtime binary itself is.
• This is a sort of mass-import of what Yury/I have been working on. It's had some rounds of pre-review in preparation, but this is all likely to be in a state of flux for some time now!

[tschneidereit]

@yurydelendik, @alexcrichton, @sunfishcode, and I have reviewed this over the last few days, so it's good to go!

[bjorn3]

Missing trailing newline

[bjorn3]

Same

[bjorn3]

Why this empty line?