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/README.md

@@ -0,0 +1,37 @@
+# `wasmtime-rust` - Using WebAssembly from Rust
+
+This crate is intended to be an example of how to load WebAssembly files from a
+native Rust application. You can always use `wasmtime` and its family of crates
+directly, but the purpose of this crate is to provide an ergonomic macro:
+
+```rust
+#[wasmtime_rust::wasmtime]
+trait WasmMarkdown {
+    fn render(&mut self, input: &str) -> String;
+}
+
+fn main() -> Result<(), failure::Error> {
+    let mut markdown = WasmMarkdown::load_file("markdown.wasm")?;
+    println!("{}", markdown.render("# Hello, Rust!"));
+
+    Ok(())
+}
+```
+
+The `wasmtime` macro defined in the `wasmtime-rust` crate is placed on a `trait`
+which includes the set of functionality which a wasm module should export. In
+this case we're expecting one `render` function which takes and returns a
+string.
+
+The macro expands to a `struct` with all of the methods on the trait (they must
+all be `&mut self`) and one function called `load_file` to actually instantiate
+the module.
+
+Note that this macro is still in early stages of development, so error messages
+aren't great yet and all functionality isn't supported yet.
+
+## Missing features
+
+Currently if the wasm module imports any symbols outside of the WASI namespace
+the module will not load. It's intended that support for this will be added soon
+though!