Parse destructuring assignment syntax

[dtolnay]

RFC: rust-lang/rfcs#2909
Tracking issue: rust-lang/rust#71126

As seen in the following test cases in rustc:

• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/drop-order.rs
• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/nested_destructure.rs
• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/slice_destructure.rs
• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/struct_destructure.rs
• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/tuple_destructure.rs
• https://github.com/rust-lang/rust/blob/cfed9184f44cbd02662eb7715d6226ae885ef5c3/src/test/ui/destructuring-assignment/tuple_struct_destructure.rs