Add support for embedding resources in the rust generator

This is relatively straight-forward via a pass in the compiler to
collect the resources to embed and then use include_bytes! (once per
resource).

What's really annoying is that the rust resource enum can't store a
&'static [u8] because cbindgen doesn't represent that, probably because
the slice representation isn't guaranteed to stay as it is. So instead
this, for now, uses raw pointers.

examples/rustwasmtest/build.rs

@@ -1,3 +1,3 @@
 fn main() {
-    sixtyfps_build::compile("src/hello.60").unwrap();
+    sixtyfps_build::compile("../cpptest/hello.60").unwrap();
 }