Add grmtools app.

grmtools (or, more specifically from this commit's perspective lrpar) is
a Yacc-compatible parser written for Rust. Although it hasn't really
been optimised, I thought this benchmark is sufficiently interesting
that users might like to know where grmtools/lrpar fits in.

I have deliberately written the lexer and parser in not only
"normal/proper Lex/Yacc style" but also "full grmtools style", including
good support for error recovery, because I think that's the only mode in
which grmtools makes sense. This might mean that this lexer/grammar is
doing a bit more work than other parsers, but since I don't expect
grmtools to be especially fast anyway, I don't suppose another few
percent slowdown will hurt!