Add helper functions for gNMI implementation.

[robshakir]

 * (M) util/*
   - Add helper functions for gNMI paths that allow implementation
     of prefix finding for a set of paths, and trimming a prefix
     from a path.
 * (M) ygot/render*.go
   - Add a render function which can take a struct value and return
     it as a gNMI typedvalue.

[coveralls]

Coverage increased (+0.02%) to 91.05% when pulling 27b4af9 on fake-api into 9867e2c on master.