Add an example language using lalrpop as a parser

[MiSawa]

This change adds an example language that uses lexgen as the lexer and lalrpop as the parser. It's in a new crate, since I wasn't able to find a way to add lalrpop code generation logic that's triggered only on build of examples/, especially without invoking some build script in all the library users' environment.

Please feel free to tweak things, change the directory structure, rename, or whatever you like (including just reject the PR of course!).