Rollup merge of #104672 - Voultapher:unify-sort-modules, r=thomcc

Unify stable and unstable sort implementations in same core module

This moves the stable sort implementation to the core::slice::sort module. By virtue of being in core it can't access `Vec`. The two `Vec` used by merge sort, `buf` and `runs`, are modelled as custom types that implement the very limited required `Vec` interface with the help of provided allocation and free functions. This is done to allow future re-use of functions and logic between stable and unstable sort. Such as `insert_head`.

This is in preparation of #100856 and #104116. It only moves code, it *doesn't* change any of the sort related logic. This unlocks the ability to share `insert_head`, `insert_tail`, `swap_if_less` `merge` and more.

Tagging ````@Mark-Simulacrum```` I hope this allows progress on #100856, by moving `merge_sort` here I hope future changes will be easier to review.

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