proc_macro/bridge: send diagnostics over the bridge as a struct
This removes some RPC when creating and emitting diagnostics, and
simplifies the bridge slightly.
After this change, there are no remaining methods which take advantage
of the support for `&mut` references to objects in the store as
arguments, meaning that support for them could technically be removed if
we wanted. The only remaining uses of immutable references into the
store are `TokenStream` and `SourceFile`.
r? `@eddyb`
This removes some RPC when creating and emitting diagnostics, and
simplifies the bridge slightly.
After this change, there are no remaining methods which take advantage
of the support for `&mut` references to objects in the store as
arguments, meaning that support for them could technically be removed if
we wanted. The only remaining uses of immutable references into the
store are `TokenStream` and `SourceFile`.
