[Accepted] SE-0507: Borrow and Mutate Accessors

[Dmitriy_Ignatyev]

@Douglas_Gregor

it is still possible to rename the mutate keyword if there is a sufficiently-motivated proposal for a better name for this accessor.

I've made a pitch for renaming: Rename borrow / mutate Accessors and Borrow / Inout Types to borrowed / mutable