lib/types: add U*_MIN and RUNE_MIN

Mirror Hare's types::limits U8_MIN..U64_MIN (all 0) and RUNE_MIN
('\0'), ref/hare/types/limits.ha:30,36,42,48,54. Pure literals,
byte-id-neutral; the U*_MIN unblock checked sat_subu* which clamp to
types.U*_MIN.

lib/types/types.ww

@@ -18,3 +18,10 @@ def U8_MAX:  u8  = 255;
 def U16_MAX: u16 = 65535;
 def U32_MAX: u32 = 4294967295;
 def U64_MAX: u64 = 18446744073709551615;
+
+def U8_MIN:  u8  = 0;
+def U16_MIN: u16 = 0;
+def U32_MIN: u32 = 0;
+def U64_MIN: u64 = 0;
+
+def RUNE_MIN: rune = '\0';