[patterns] Destructuring assignment, clarify patterns in for, etc. (#2449)

* [patterns] Destructuring assignment, clarify patterns in for, etc.

- Add destructuring assignment. Fix #2438.
- Specify context type for empty list and map patterns. Fix #2441.
- Define grammar for outermost patterns in a declaration. Fix #2446.
- Rename "grouping" to "parenthesized" patterns. Fix #2447.
- Specify behavior of patterns in for loops better. Fix #2448.

* Make null-check and logical-or patterns irrefutable.

Disallow assigning to the same variable multiple times in an assignment.

* Apply review feedback.

Author: munificent