Rollup merge of #76655 - CDirkx:const-pin, r=ecstatic-morse

Make some methods of `Pin` unstable const

Make the following methods unstable const under the `const_pin` feature:
- `new`
- `new_unchecked`
- `into_inner`
- `into_inner_unchecked`
- `get_ref`
- `into_ref`
- `get_mut`
- `get_unchecked_mut`

Of these, `into_inner` and `into_inner_unchecked` require the unstable `const_precise_live_drops`.

Also adds tests for these methods in a const context.

Tracking issue: #76654

r? @ecstatic-morse

library/core/tests/lib.rs

@@ -43,6 +43,8 @@
 #![feature(iter_order_by)]
 #![feature(cmp_min_max_by)]
 #![feature(iter_map_while)]
+#![feature(const_mut_refs)]
+#![feature(const_pin)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_raw_ptr_deref)]
 #![feature(never_type)]
@@ -79,6 +81,7 @@ mod num;
 mod ops;
 mod option;
 mod pattern;
+mod pin;
 mod ptr;
 mod result;
 mod slice;