Rollup merge of #48527 - zackmdavis:and_the_social_construction_of_tuples, r=estebank

in which parentheses are suggested for should-have-been-tuple-patterns

![destructure_suggest_parens](https://user-images.githubusercontent.com/1076988/36638335-48b082d4-19a7-11e8-9726-0d043544df2f.png)

Programmers used to working in some other languages (such as Python or
Go) might expect to be able to destructure values with comma-separated
identifiers but no parentheses on the left side of an assignment.

Previously, the first name in such code would get parsed as a
single-indentifier pattern—recognizing, for example, the
`let a` in `let a, b = (1, 2);`—whereupon we would have a fatal syntax
error on seeing an unexpected comma rather than the expected semicolon
(all the way nearer to the end of `parse_full_stmt`).

Instead, let's look for that comma when parsing the pattern, and if we
see it, make-believe that we're parsing the remaining elements in a
tuple pattern, so that we can suggest wrapping it all in parentheses. We
need to do this in a separate wrapper method called on a "top-level"
pattern, rather than within
`parse_pat` itself, because `parse_pat` gets called recursively to parse
the sub-patterns within a tuple pattern.

~~We could also do this for `match` arms, `if let`, and `while let`, but
we elect not to in this patch, as it seems less likely for users to make
the mistake in those contexts.~~

Resolves #48492.

r? @petrochenkov

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