Implement a more complete parameter list grammar

[arnsholt]

The reference grammar is kind of tangled in this particular area, but I think the actual grammar goes something like this:

parameter_list: <positionals>? <positional_slurpy>? <nameds>? <named_slurpy>?

Where positionals is further constrained to not allow arguments without defaults once an argument with a default has been seen, and a positional slurpy can be either an identifier prefixed by * or just a literal star (signifying the end of the positional list without a slurpy).

There may be some further shenanigans required to be able to pass positionals by name and such, but this should at least get us a lot closer than we currently are.

[arnsholt]

Incidentally, a nice way to test named arguments and slurpies would be to get enough of the scaffolding in place to implement a better print, with arbitrary arguments and the sep and end arguments.