parser: factor out function that parses right-hand side of prefix/infix ops
These two had common pattern. The significant difference was just a result
expression:
prefix: (op-name, rhs)
infix: (op-name, lhs, rhs)
http://example.com/no/anchor, branches: (None, [])
http://example.com/an/anchor, branches: ('foo', [])
http://example.com/no/anchor/branches, branches: (None, ['foo'])
http://example.com/an/anchor/branches, branches: ('bar', ['foo'])
http://example.com/an/anchor/branches-None, branches: ('foo', [])
http://example.com/, branches: (None, [])
http://example.com/, branches: (None, [])
http://example.com/, branches: ('foo', [])