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/src/lib.rs

@@ -0,0 +1,47 @@
+pub use wasmtime_rust_macro::wasmtime;
+
+// modules used by the macro
+#[doc(hidden)]
+pub mod __rt {
+    pub use cranelift_codegen;
+    pub use cranelift_native;
+    pub use failure;
+    pub use wasmtime_interface_types;
+    pub use wasmtime_jit;
+
+    use std::convert::{TryFrom, TryInto};
+    use wasmtime_interface_types::Value;
+
+    pub trait FromVecValue: Sized {
+        fn from(list: Vec<Value>) -> Result<Self, failure::Error>;
+    }
+
+    macro_rules! tuple {
+        ($(($($a:ident),*),)*) => ($(
+            impl<$($a: TryFrom<Value>),*> FromVecValue for ($($a,)*)
+                where $(failure::Error: From<$a::Error>,)*
+            {
+                #[allow(non_snake_case)]
+                fn from(list: Vec<Value>) -> Result<Self, failure::Error> {
+                    let mut iter = list.into_iter();
+                    $(
+                        let $a = iter.next()
+                            .ok_or_else(|| failure::format_err!("not enough values"))?
+                            .try_into()?;
+                    )*
+                    if iter.next().is_some() {
+                        failure::format_err!("too many return values");
+                    }
+                    Ok(($($a,)*))
+                }
+            }
+        )*)
+    }
+
+    tuple! {
+        (),
+        (A),
+        (A, B),
+        (A, B, C),
+    }
+}