Idea: Pre-expanded crates and `expansion-dependencies`

[djc]

See recent discussion here:

Pre-expanding proc macros

That would require a special case in every build system to check if rustc supports wasm proc macros on the current target and if not switch to compiling native dylibs. It would also mean that we can't remove the -Zdual-proc-macro hack that is necessary for cross compiling rustc, but breaks cross compiling tools linked against rustc itself.