This pretty much reworks the entire icon system. Some notable changes:
* The extensions are put into a statically generated hash-table instead
of a sorted array. We use Robin-Hood insertion to reduce the max probe
length. Currently we need to probe only 2 slots for `O_EMOJI` and only
3 for `O_NERD`/`O_ICONS`.
* I've opted not to use a perfect-hash since the perfect hashes
generated by [`gperf`](https://www.gnu.org/software/gperf) used some
huge lookup table. The hash function also wasn't as minimal as I'd
like.
* Colors are now using X-Macros. This should speed up startup since we
don't have to search `icons_ext` linearly to find unique colors.
* The hash-table generator outputs a more space optimized `struct
icon_pair` using a char array instead of char pointer. This brings
down the binary size from `145KiB` when using `O_NERD` down to
`137KiB`.
* Some unnecessary duplication and indirection has been reduced by using
the `ICON_STR()` macro.
* Branched v4.2
Added workaround for Mac OS X < 10.12.0
(Only tested on 10.11.6, lower versions may need more workaround)
* Added *.dSYM to .gitignore
* Added comments for the macosx detection in Makefile
* Fixed indentation, formatting and missing newline at eof
* Moved includes inside include guard
Co-authored-by: elder-n00b <elder-n00b@yandex.com>
