Make mir-borrowck more closely match (draft) NLL RFC.

In particular:

 * introduce the shallow/deep distinction for read/write accesses

 * use the notions of prefixes, shallow prefixes, and supporting prefixes
   rather than trying to recreate the restricted sets from ast-borrowck.

 * Add shallow reads of Discriminant and ArrayLength, and treat them
   as artificial fields when doing prefix traversals.

Author: pnkfelix