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/cond.h

@@ -91,5 +91,5 @@ public:
 		}
 	}
 
-	static constexpr u64 max_timeout = UINT64_MAX / 1000;
+	static constexpr u64 max_timeout = u64{umax} / 1000;
 };