The crtdbg allocators are currently being implemented as inline
functions as part of the "w32_crtdbg_stacktrace.h" header. As we are
moving towards pluggable allocators with the help of function pointers,
though, we cannot make use of inlining anymore. Instead, we can only
have a single implementation of these allocating functions.

Move all implementations of the crtdbg allocators into
"w32_crtdbg_stacktrace.c".