lib: parser: unify subgraphs & simplify even more

lib: parser: unify subgraphs & simplify even more

This cuts a large piece of complexity from the parser by making the
sequences between | the same, no matter whether it's <> () or [].

This causes some changes in behaviour:
- [foo|bar] is now accepted
- <foo|[bar]> is now accepted (yes it's stupid)
- {a|b} is now means "at least one of these, in any order"; to allow
  zero use [{a|b}] instead.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>