Require that the values to the path element properties are numerical literals.
The conversion happens in the path "compilation" pass, where we can do more
pre-calcuation in the future.
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.
The LoweredItem and LoweredComponent contained, in essence, the same
information as the Element and Component in object_tree. Since the
moving declarations pass moved everything to the root element and the
LoweredPropertyDeclarations have been removed as well, this is the last
step.
