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/src/lib.rs

@@ -78,6 +78,7 @@
 #![feature(const_int_pow)]
 #![feature(constctlz)]
 #![feature(const_panic)]
+#![feature(const_pin)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]
 #![feature(const_option)]