Implement &pin patterns and ref pin binding modes

[frank-king]

Implement &pin patterns and ref pin binding modes, part of pin ergonomics.

[rust-log-analyzer]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] askama test:false 0.172
error[E0023]: this pattern has 2 fields, but the corresponding tuple variant has 3 fields
    --> src/librustdoc/clean/utils.rs:306:40
     |
 306 |         PatKind::Box(p) | PatKind::Ref(p, _) | PatKind::Guard(p, _) => return name_from_pat(p),
     |                                        ^  ^ expected 3 fields, found 2
     |
    ::: /checkout/compiler/rustc_hir/src/hir.rs:1918:9
     |
1918 |     Ref(&'hir Pat<'hir>, Pinnedness, Mutability),
     |         ---------------  ----------  ---------- tuple variant has 3 fields
     |
help: use `_` to explicitly ignore each field
     |
 306 |         PatKind::Box(p) | PatKind::Ref(p, _, _) | PatKind::Guard(p, _) => return name_from_pat(p),
     |                                            +++

For more information about this error, try `rustc --explain E0023`.
[RUSTC-TIMING] rustdoc test:false 3.616
error: could not compile `rustdoc` (lib) due to 1 previous error