Rollup merge of #74333 - poliorcetics:std-alloc-unsafe-op-in-unsafe-fn, r=LukasKalbertodt

Deny unsafe operations in unsafe functions in libstd/alloc.rs

Partial fix of #73904.

This encloses `unsafe` operations in `unsafe fn` in `libstd/alloc.rs`.

@rustbot modify labels: F-unsafe-block-in-unsafe-fn