rules: rewrite

Rewrite the rules file parser for clarity, performance and memory usage
reduction. The previous implementation was quite hard to navigate and
did a lot of unnecessary work and copying.
This implementation keeps along just the state necessary, and doesn't
perform any copying of the file's content (although the entire file is
mmap'ed as before). Hopefully it's also easier to understand, has better
documentation, and better error checking and reporting. We try to
reproduce the previous behavior in every case.

Note: the diff is pretty confusing; it's likely better to look at the
file directly.

Benchmarks:
    On an old 32-bit Intel processor.
    gcc -O2 -pg
    ./test/rulescomp bench
    grof test/rulescomp

    Before:
        compiled 1000 keymaps in 14.863564304s
        %   cumulative   self              self     total
        time   seconds   seconds    calls  ms/call  ms/call  name
        49.33      4.43     4.43 30610000     0.00     0.00  yylex
        17.93      6.04     1.61    31000     0.05     0.22  yyparse
        6.57      6.63     0.59     1000     0.59     0.59  load_rules
        3.23      6.92     0.29  3637000     0.00     0.00  AppendStmt
        2.45      7.14     0.22   472000     0.00     0.00  AddKeySymbols
        2.12      7.33     0.19  3591000     0.00     0.00  atom_intern
        2.12      7.52     0.19   518000     0.00     0.00  FindNamedKey
        2.00      7.70     0.18   230000     0.00     0.00  FreeStmt
        1.78      7.86     0.16     1000     0.16     0.17  UpdateModifiersFromCompat
        1.34      7.98     0.12   732000     0.00     0.00  AddKeyName
        1.34      8.10     0.12                             __x86.get_pc_thunk.bx

    After:
        compiled 1000 keymaps in 13.874666269s
        %   cumulative   self              self     total
        time   seconds   seconds    calls  ms/call  ms/call  name
        49.82      4.26     4.26 30610000     0.00     0.00  yylex
        22.22      6.16     1.90    31000     0.06     0.22  yyparse
        2.92      6.41     0.25  3591000     0.00     0.00  atom_intern
        2.57      6.63     0.22     1000     0.22     0.25  xkb_components_from_rules
        2.11      6.81     0.18  3637000     0.00     0.00  AppendStmt
        2.11      6.99     0.18   230000     0.00     0.00  FreeStmt
        1.99      7.16     0.17   518000     0.00     0.00  FindNamedKey
        1.99      7.33     0.17     1000     0.17     0.17  UpdateModifiersFromCompat
        1.99      7.50     0.17                             __x86.get_pc_thunk.bx
        1.52      7.63     0.13   150000     0.00     0.00  AddInterp
        1.40      7.75     0.12   472000     0.00     0.00  AddKeySymbols

    On a newer 64-bit Intel processor.
    gcc -O2
    ./test/rules-file bench

    Before:
        processed 20000 times in 15.940546625s

    After:
        processed 20000 times in 5.295026345s

Allocations:
    gcc -O2
    valgrind test/rulescomp

    Before:
    total heap usage: 257,519 allocs, 257,519 frees, 14,766,529 bytes allocated

    After:
    total heap usage: 240,756 allocs, 240,756 frees, 14,007,886 bytes allocated

Signed-off-by: Ran Benita <ran234@gmail.com>
rules: use goto instead of state variable

There's no noticeable speed difference, but I think it's nicer and more
explicit than the previous code. Some people just don't like goto,
though..

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