more mangling for std::allocator

This makes calling std::allocator.deallocator() work.

Interfacing with C++ templates is never going to be pretty, but it seems we can make it work.
I think I found a complete list of all mangling compression tricks used by gcc. See http://www.open64.net/doc/d9/d6e/kg_09_09fe_2gnu_2cp_2mangle_8c-source.html. There's an `enum` there with `SUBID_ALLOCATOR` and others, which are, it seems, all special cases of mangling.