Add PathStruct definition and child ctor generating code for ypathgen

[wenovus]

This is the first step to a generating library (ypathgen) alongside ygen
which generates an API for creating YANG paths given a set of YANG
schema files. While ygen creates a set of structs for storing the data
of a YANG tree, ypathgen creates a set of structs for generating paths
denoting nodes in a YANG tree. This avoids any manual construction of
paths.

Envisioned API example:
oc.Interface("eth0").Counters().InOctets()

Each selector constructs and returns a child struct that keeps track of
the absolute path it represents.

This change includes the generation of these child constructors.

List of primary changes:

• ygot/path_types.go defines the base struct and interface to be
  implemented by every path struct node.
• KeyValueAsString() is enhanced to allow ygen.Binary to be converted to
  a string for use by gNMI as a list key value.
• A GetDirectories() call is added to ygen for retrieving a parsed map of
  Directory elements for use by the ypathgen generation library. The
  output of this function is very similar to part of what ygen uses for
  its generation.
• The configuration struct for ygen code generation is reorganized.
• pathgen.go contains the child path struct constructor generation code.

[coveralls]

Coverage decreased (-0.2%) to 90.803% when pulling a930971 on wenbli-ypathgen-childctor into 5b5bc6b on master.

[robshakir]

Thanks for the change here Wen -- LGTM, please go ahead and merge!