Trailing comma

[kddnewton]

Fixed a couple of inconsistencies with the parse.y grammar:

• Trailing comma on jumps at EOF — return a, / break a, / next a, are now rejected.
• Trailing comma on parenthesized yield — yield(a,) rejected
• Non-associativity at the same binding power — 1...2..3, a == b != c (different non-assoc operators sharing a precedence) are now rejected, not just same-token chains.
• Trailing comma in one-line pattern matches — x => a, / foo in bar, now parse as single-element array patterns
• Trailing comma after ... forwarding — foo(...,) rejected.
• Trailing comma after a rest in array patterns — [a, *b,] rejected (second-rest error)