Add an example #[wasmtime] Rust macro (#283)

This commit adds a `wasmtime-rust` crate to the `misc` folder next to
the previously added Python extension. The intention is that this
showcases loading a WebAssembly file natively in Rust and how with an
attribute macro it can feel lightweight in terms of boilerplate.

The macro itself is pretty non-featureful today beyond the bare bones to
get anything working, but there's all sorts of possibilities like
JIT-compiled entry stubs we could eventually do with all the type
information!

Cargo.toml

@@ -39,7 +39,10 @@ rayon = "1.1"
 wasm-webidl-bindings = "0.4"
 
 [workspace]
-members = ["misc/wasmtime-py"]
+members = [
+  "misc/wasmtime-rust",
+  "misc/wasmtime-py",
+]
 
 [features]
 lightbeam = ["wasmtime-environ/lightbeam", "wasmtime-jit/lightbeam"]