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!

misc/wasmtime-rust/Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "wasmtime-rust"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+edition = "2018"
+
+[lib]
+test = false
+doctest = false
+
+[dependencies]
+cranelift-codegen = "0.38.0"
+cranelift-native = "0.38.0"
+failure = "0.1.5"
+wasmtime-interface-types = { path = "../../wasmtime-interface-types" }
+wasmtime-jit = { path = "../../wasmtime-jit" }
+wasmtime-rust-macro = { path = "./macro" }