Standard library ptr routines cleanup (#470)
Standard library ptr routines cleanup (#470)

* Updated proposal:

- removing `withUnsafe[Mutable]Pointers`
- keeping `withUnsafe[Mutable]Pointer` arg as `inout`

* Updating proposal to include requested change by the core team.

* Fixing missing end of sentence.

* Fixing a typo.

* Another typo fix.

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