Make enums unsigned

Speed up by using unsigned enums.

Passed STC:
LLR: 2.98 (-2.94,2.94) <0.00,2.00>
Total: 49248 W: 12894 L: 12568 D: 23786
Ptnml(0-2): 119, 5353, 13397, 5593, 16
https://tests.stockfishchess.org/tests/view/695e3e5002d0182a589fe965

closes https://github.com/official-stockfish/Stockfish/pull/6532

No functional change

src/timeman.h

@@ -26,7 +26,7 @@
 namespace Stockfish {
 
 class OptionsMap;
-enum Color : int8_t;
+enum Color : uint8_t;
 
 namespace Search {
 struct LimitsType;