Rollup merge of #148755 - nxsaken:const_drop_guard, r=dtolnay

Constify `DropGuard::dismiss` and trait impls

Feature: `drop_guard` (rust-lang/rust#144426), `const_convert` (rust-lang/rust#143773), `const_drop_guard` (no tracking issue yet)

Constifies `DropGuard::dismiss` and trait impls.
I reused `const_convert` (rust-lang/rust#143773) for the `Deref*` impls.

Author: ChrisDenton