Potential optimizations

[SichangHe]

Dumping the optimizations that could speed up the route_verification_bgp. I am not interested in implementing them, though, because the current implementation is fast enough for our needs.

• Distinguish IPv4Net and IPv6Net in AsRoute. This would be a significant memory save and potentially visible speedup.
• Cache the AS-path regex interpreters. This would be little to no speedup since very few regexes are used.
• Optimize some of the special case checks that run recursively.

There are also nicety improvements:

• Have source map in the AST. This would pretty much require a rewrite of the lexer.