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.

api/sixtyfps-cpp/include/sixtyfps_resource.h

@@ -7,6 +7,10 @@ namespace sixtyfps {
 
 union ResourceData {
     SharedString absolute_file_path;
+    struct {
+        void *ptr;
+        size_t len;
+    } slice;
 
     ResourceData() { }
     ~ResourceData() { }