keymap: add xkb_keymap_new_from_memory()

The current API doesn't allow the caller to create keymaps from mmap()'ed
files. The problem is, xkb_keymap_new_from_string() requires a terminating
0 byte. However, there is no way to guarantee that when using mmap() so a
user currently has to copy the whole file just to get the terminating zero
byte (assuming they cannot use xkb_keymap_new_from_file()).

This adds a new entry xkb_keymap_new_from_memory() which takes a memory
location and the size in bytes.

Internally, we depend on yy_scan_{string,byte}() helpers. According to
flex documentation these already copy the input string because they are
wrappers around yy_scan_buffer().
yy_scan_buffer() on the other hand has some insane requirements. The
buffer must be writeable and the last two bytes must be ASCII-NUL. But the
buffer may contain other 0 bytes just fine.

Because we don't want these constraints in our public API,
xkb_keymap_new_from_memory() needs to create a copy of the input memory.
But it then calls yy_scan_buffer() directly. Hence, we have the same
number of buffer-copies as with *_from_string() but without the
terminating 0 requirement.

Maybe some day we no longer depend on flex and can have a zero-copy API. A
user could mmap() a file and it would get parsed right from this buffer.
But until then, we shouldn't expose this limitation in the API but instead
provide an API that some day can work with zero-copy.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

[ran: rebased on top of my branch]
Conflicts:
	Makefile.am
	src/xkbcomp/xkbcomp.c
Replace flex scanner with a hand-written one

The scanner is very similar in structure to the one in xkbcomp/rules.c
(might be able to share some code there). 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>