Parser fixes for parenthesized groups

[anweiss]

Fixes issue parsing parenthesized groups and includes some if let cleanup. Now properly parses the following example:

CRI-Reference = [
  (?scheme, ?((host.name // host.ip), ?port) // path.type),
  *path,
  *query,
  ?fragment
]

scheme = (0, text .regexp "[a-z][a-z0-9+.-]*")
host.name = (1, text)
host.ip = (2, bytes .size 4 / bytes .size 16)
port = (3, 0..65535)
path.type = (4, 0..127)
path = (5, text)
query = (6, text)
fragment = (7, text)