Implement `&P` patterns for borrowed ptrs

[graydon]

See conclusion of the saga of unsafe pointers trying on caret:

#2826 (comment)

Essentially we decided to leave & as borrowed-ptr, and * as unsafe, and just add a keyword for capture-by-reference since we expect it'll be a rare-ish occurrence in the pattern language.

(or * rather than ref. neither is in the pattern grammar at present. pick one.)

[bblum]

Part of #3271. Isn't this already done, or is there some borrowck integration left?

[bblum]

oops didn't mean to close it.

[nikomatsakis]

This is basically done, except that there is no pattern for borrowed ptrs.

[catamorphism]

I'll take this over.

[bblum]

is there something concrete left to do for this, apart from the "make the ref keyword work more smoothly" problems? (#3235, #3256)

[nikomatsakis]

Yes, we need to implement & patterns. I will update the subject to be more specific.