winstl::squeeze_path() redux

As of 1.9.69, the winstl::squeeze_path() function template is much improved, mainly due to extensive unit-testing. This, in turn, is a fall-out of the the removal of dependencies on so-called "unsafe" string functions, such as strncpy(), strcat(), and so forth. And that, in turn, is due to a "need" to be compatible with Microsoft's perverse designation of standard library functions as deprecated. Ho hum.

Anyway, the improvements now mean that it's robust for all buffer sizes between [0, strlen(path)] and beyond. So, you can do all of the following:

std::string path = "H:\\xyz\\mno\\abcdef.ghi";
char buffer[101];

// returns the number required (22)
winstl::path_squeeze(path, static_cast(NULL), 0);

// returns 1; buffer == ""
winstl::path_squeeze(path, buffer, 1);

// returns 2; buffer == "a"
winstl::path_squeeze(path, buffer, 2);

// returns 5; buffer == "abcd"
winstl::path_squeeze(path, buffer, 5);

// returns 6; buffer == "a...i"
winstl::path_squeeze(path, buffer, 6);

// returns 9; buffer == "ab...ghi"
winstl::path_squeeze(path, buffer, 9);

// returns 10; buffer == "abc...ghi"
winstl::path_squeeze(path, buffer, 10);

// returns 11; buffer == "abcdef.ghi"
winstl::path_squeeze(path, buffer, 11);

// returns 11; buffer == "abcdef.ghi"
winstl::path_squeeze(path, buffer, 12);

// returns 11; buffer == "abcdef.ghi"
winstl::path_squeeze(path, buffer, 17);

// returns 18; buffer == "H:\\...\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 18);

// returns 19; buffer == "H:\\x...\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 19);

// returns 20; buffer == "H:\\xy...\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 20);

// returns 21; buffer == "H:\\xyz...\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 21);

// returns 22; buffer == "H:\\xyz\\mno\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 22);

// returns 22; buffer == "H:\\xyz\\mno\\abcdef.ghi"
winstl::path_squeeze(path, buffer, 23);