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!

wasmtime-interface-types/src/value.rs

@@ -1,3 +1,4 @@
+use std::convert::TryFrom;
 use std::fmt;
 
 /// The set of all possible WebAssembly Interface Types
@@ -20,6 +21,17 @@ macro_rules! from {
                 Value::$b(val)
             }
         }
+
+        impl TryFrom<Value> for $a {
+            type Error = failure::Error;
+
+            fn try_from(val: Value) -> Result<$a, Self::Error> {
+                match val {
+                    Value::$b(v) => Ok(v),
+                    v => failure::bail!("cannot convert {:?} to {}", v, stringify!($a)),
+                }
+            }
+        }
     )*)
 }