types.hpp: implement smin, smax, amin, amax

Rewritten the following global utility constants:
`umax` returns max number, restricted to unsigned.
`smax` returns max signed number, restricted to integrals.
`smin` returns min signed number, restricted to signed.
`amin` returns smin or zero, less restricted.
`amax` returns smax or umax, less restricted.

Fix operators == and <=> for synthesized rel-ops.

Utilities/File.cpp

@@ -97,7 +97,7 @@ static FILETIME from_time(s64 _time)
 		result.dwLowDateTime = 0;
 		result.dwHighDateTime = 0;
 	}
-	else if (_time > INT64_MAX / 10000000ll - 11644473600ll)
+	else if (_time > s64{smax} / 10000000ll - 11644473600ll)
 	{
 		result.dwLowDateTime = 0xffffffff;
 		result.dwHighDateTime = 0x7fffffff;