Jcmp 1.2.23.0

Changed (breaking): int is now a 32-bit integer (i32 is another name of it) and i64 is the 64-bit integer; int + int wraps at 32 bits. python3 tools/int2i64.py converts a program that needs 64 bits (docs/MIGRATION.md).
Added: i64 as a type name, t609. Fixed: Span and Span were different instances.