The current order of declarations and includes between "common.h" and
"w32_crtdbg_stacktrace.h" is rather complicated. Both header files make
use of things defined in the other one and are thus circularly dependent
on each other. This makes it currently impossible to compile the
"w32_crtdbg_stacktrace.c" file when including "common.h" inside of
"w32_crtdbg_stacktrace.h".

We can disentangle the mess by moving declaration of the inline crtdbg
functions into the "w32_crtdbg_stacktrace.h" file and adding additional
includes inside of it, such that all required functions are available to
it. This allows us to break the dependency cycle.