Replace flex scanner with a hand-written one

Replace flex scanner with a hand-written one

The scanner is very similar in structure to the one in xkbcomp/rules.c.
It avoids copying and has nicer error reporting.

It uses gperf to generate a hashtable for the keywords, which gives a
nice speed boost (compared to the naive strcasecmp method at least). But
since there's hardly a reason to regenerate it every time and require
people to install gperf, the output (keywords.c) is added here as well.

Here are some stats from test/rulescomp:

Before:
compiled 1000 keymaps in 4.052939625s
==22063==   total heap usage: 101,101 allocs, 101,101 frees, 11,840,834 bytes allocated

After:
compiled 1000 keymaps in 3.519665434s
==26505==   total heap usage: 99,945 allocs, 99,945 frees, 7,033,608 bytes allocated

Signed-off-by: Ran Benita <ran234@gmail.com>