Module symbol

Client-side interner used for symbols.

This is roughly based on the symbol interner from rustc_span and the DroplessArena from rustc_arena. It is unfortunately a complete copy/re-implementation rather than a dependency as it is difficult to depend on crates from within proc_macro, due to it being built at the same time as std.

If at some point in the future it becomes easier to add dependencies to proc_macro, this module should probably be removed or simplified.