implement `ISASCII` correctly
`ISSPACE` evaluates its argument multiple times
don't (re-)define `ascii_isspace` and `isascii`

`ISSPACE` and `ISASCII` can be used instead.
use uppercase US-ASCII ctype macros
partially revert 1a5841b for mirb

Fix compilation fail caused by GNU Readline and `ISPRINT`.
Also add missing `unsigned char` cast.
Merge branch 'ascii' of https://github.com/cremno/mruby into cremno-ascii
git push origin masterMerge branch 'cremno-ascii'