Rework time functions a bit

- mktime: Work unsigned as time_t's type implies (shifting Y2K38 bug to 2106)
- mktime: Add unit tests
- gmtime/localtime: factorize
- gmtime/localtime: Add unit tests
- mktime/gmtime/localtime: Size optimisation (-130 bytes wrt master)
- mktime: Speed optimisation (from 23M cycles on the unit test to 2M)

libsrc/common/_is_leap_year.h

@@ -0,0 +1,22 @@
+/*
+** _is_leap_year.h
+**
+** (C) Copyright 2024, Colin Leroy-Mira <colin@colino.net>
+**
+*/
+
+
+
+#ifndef __IS_LEAP_YEAR_H
+#define __IS_LEAP_YEAR_H
+
+
+
+unsigned char __fastcall__ IsLeapYear (unsigned char Year);
+/* Returns 1 if the given year is a leap year. Expects a year from 0 to 206,
+ * without 1900 added */
+
+
+
+/* End of _is_leap_year.h */
+#endif