Make _modify public

[Jean-Daniel]

Modify Accessors

Hi Swift Evolution, Some holiday reading for you, a pitch of a new way of writing mutable computed properties and subscripts. For more information on the performance implications of this change and why it's an important feature to add, you might also find this talk from last year's Functional Swift Conference useful. Proposal We propose the introduction of a new keyword, modify, for implementing mutable computed properties and subscripts, alongside the current get and set. The bodies of mod…