Add support for trailing commas in more places

This lets the parser understand trailing commas in method calls, method
definitions, enum variants, and type parameters.

Closes #14240.
Closes #15887.
auto merge of #16646 : P1start/rust/trailing-commas, r=alexcrichton

This lets the parser understand trailing commas in method calls, method definitions, enum variants, and type parameters.

Closes #14240.
Closes #15887.