proposal for buffer and pointer initialization improvements

[glessard]

• as pitched in https://forums.swift.org/t/53168

[glessard]

I wonder if this should be merged with the "partial initialization" pitch (thread). Most of the extensions in the Slice pitch overlap with this, as it reproduces the functions on buffers.

[atrick]

I wonder if this should be merged with the "partial initialization" pitch (thread). Most of the extensions in the Slice pitch overlap with this, as it reproduces the functions on buffers.

FWIW, I actually didn't realize there were two separate initialization API proposals. I've been confusing them with each other all along.

[glessard]

Closing this PR in favor of #1561