mangle C++ ::std and ::std::allocator correctly

On Linux, there are shortcut mangles for ::std and ::std::allocator which were not implemented.

Not done on Windows because C++ template mangling currently isn't done at all on Windows. Need to fix that with another PR.

Also wound up removing some tabs from the test files.
I did not do that because there's potentially more things that can make std qualified or not than simply isConst - volatile, restricted, and ref apply, according to the ABI spec.