Stabilize `get_many_mut` as `get_disjoint_mut`

* Renames the methods:
	* `get_many_mut` -> `get_disjoint_mut`
	* `get_many_unchecked_mut` -> `get_disjoint_unchecked_mut`
* Does not rename the feature flag: `get_many_mut`
* Marks the feature as stable
* Renames some helper stuff:
	* `GetManyMutError` -> `GetDisjointMutError`
	* `GetManyMutIndex` -> `GetDisjointMutIndex`
	* `get_many_mut_helpers` -> `get_disjoint_mut_helpers`
	* `get_many_check_valid` -> `get_disjoint_check_valid`

This only touches slice methods.
HashMap's methods and feature gates are not renamed here
(nor are they stabilized).