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/examples/markdown.rs

@@ -0,0 +1,13 @@
+use wasmtime_rust::wasmtime;
+
+#[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(())
+}