Auto merge of #147893 - fee1-dead-contrib:constheapheapheap, r=oli-obk

`Vec::push` in consts MVP

Example:

```rust
const X: &'static [u32] = {
    let mut v = Vec::with_capacity(6);
    let mut x = 1;
    while x < 42 {
        v.push(x);
        x *= 2;
    }
    assert!(v.len() == 6);
    v.const_make_global()
};

assert_eq!([1, 2, 4, 8, 16, 32], X);
```

Oh this is fun...

* We split out the implementation of `Global` such that it calls `intrinsics::const_allocate` and `intrinsics::const_deallocate` during compile time. This is achieved using `const_eval_select`
* This allows us to `impl const Allocator for Global`
* We then constify everything necessary for `Vec::with_capacity` and `Vec::push`.
* Added `Vec::const_make_global` to leak and intern the final value via `intrinsics::const_make_global`. If we see any pointer in the final value of a `const` that did not call `const_make_global`, we error as implemented in rust-lang/rust#143595.

r? `@rust-lang/wg-const-eval`

To-do for me:
* [x] Assess the rustdoc impact of additional bounds in the method
* [x] ~~Increase test coverage~~ I think this is enough for an unstable feature.

Author: bors