syntax: Allow 1-tuple expressions

r? @graydon - This is for greater uniformity (for example, macros that generate
tuples). rustc already supported 1-tuple patterns, but there was no
way to construct a 1-tuple term.

@graydon , as far as your comment on #4898 - it did turn out to be solvable inside the macro (since @luqmana already fixed it using structs instead), but I still think it's a good idea to allow 1-tuples, for uniformity. I don't think anyone is likely to trip over it, and I'm not too worried that it changes the amount of ambiguity.