Avoid confusion with the GNU extension 'program_invocation_name', which
is described by 'man 3 program_invocation_name'. The GNU version isn't
supposed to be exposed without defining _GNU_SOURCE, which we don't in
any of the relevant files, but it's best to avoid any confusion.
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.
QueryPerformanceCounter() is much more accurate than
GetSystemTimeAsFileTime().
Also avoid converting into a specific time unit until the results need
to be displayed. This is more efficient and avoids losing precision.
