The Windows-specific crtdbg allocator is currently mixed into the
crtdbg stacktracing compilation unit, making it harder to find
than necessary. Extract it and move it into the new "allocators/"
subdirectory to improve discoverability.

This change means that the crtdbg compilation unit is now
compiled unconditionally, whereas it has previously only been
compiled on Windows platforms. Thus we now have additional guards
around the code so that it will only be compiled if
GIT_MSVC_CRTDBG is defined. This also allows us to move over the
fallback-implementation of `git_win32_crtdbg_init_allocator` into
the same compilation unit.