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.s

@@ -0,0 +1,23 @@
+;
+; Colin Leroy-Mira, 2024
+;
+; unsigned char __fastcall__ IsLeapYear (unsigned char Year)
+; Returns 1 in A if the given year is a leap year. Expects a year from 0 to 206,
+; without 1900 added.
+;
+
+        .export         _IsLeapYear
+
+_IsLeapYear:
+        ldx     #$00            ; Prepare X for rts
+        cmp     #$00            ; Y 0 (1900) is not a leap year
+        beq     NotLeap
+        cmp     #$C8            ; Y 200 (2100) is not a leap year
+        beq     NotLeap
+        and     #$03            ; Year % 4 == 0 means leap year
+        bne     NotLeap
+        lda     #$01            ; Return 1
+        rts
+NotLeap:
+        lda     #$00            ; Return 0
+        rts