Generate "Map" PathStructs and gNMI helpers

[wenovus]

These are PathStructs that are unique to keyed-list types that refer to
the entire map object representing the list.

* This only pertains to compressed structs (will not affect uncompressed
  PathStruct generation).
* Both non-wildcard and wildcard versions are generated. For Wildcard []map will
  be the return type.
* Refactored the code to handle atomic lists as well without impacting ordered map
  generation. Child constructors for atomic lists are also cut off similar to ordered maps.

Tests are in following PR.

Comments to help with reviewing this change:

• Non-atomic lists have both regular PathStructs and "Map" PathStructs generated.
• Atomic (incl. ordered) lists have only "Map" PathStructs generated.
• Due to the above, only the corresponding NodeData are generated.

[coveralls]

Pull Request Test Coverage Report for Build 5361454072

• 83 of 93 (89.25%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 84.098%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pathgen/pathgen.go	72	82	87.8%
<!--	Total:	83	93	89.25%	-->

Files with Coverage Reduction	New Missed Lines	%
pathgen/gnmigen.go	1	82.35%
<!--	Total:	1		-->

Totals
Change from base Build 5351195132:	0.2%
Covered Lines:	1777
Relevant Lines:	2113

---

💛 - Coveralls