Clean up:
- replaced m1ri_word (const) with M1RI_RADIX (define)
- removed hardcoded 64 and replaced by M1RI_RADIX
- prefixed macros with M1RI_ (namespace-ish)
- made iswindowed an actual flag
- m1ri_die() added, don't call exit(1) in a library
- some indentation changed, should be unified