Not all programming languages support the cdecl calling convention.
stdcall is what the Win32 API uses, and it seems to be the better choice
for maximum compatibility with other programming languages.
So, switch from cdecl to stdcall.
Resolves https://github.com/ebiggers/libdeflate/issues/58
Move program utility functions that are used only by "test programs"
(i.e. not by gzip/gunzip) from prog_util.{c,h} into test_util.{c,h}.
This reduces the code that is compiled for the default build target,
which excludes the test programs.
