Refactor parser.c for better testability

[brawner]

This PR separates out the parser.c functionality into separate source files with their own headers so additional unit tests can be added to target specific functions. I still need to create the additional unit tests, but this current refactor passes the tests that currently exist, which has about 88% coverage.

Because this PR is so large, I've separated the changes into logical commits. Reviewing each commit individually might be the easiest way to scan through this PR as they illustrate the copy-paste changes I did. They could be their own PRs if that's desired by the reviewers.

Main commits

• 8872020 Shuffles functions into their own separate files, but doesn't change any actual logic
• 70e0c99 Reorders parser.c to match parser.h
• bbb23d2 Creates a couple of macros and reduces some of the duplicate code when copying rcl_yaml_variant_t
• 5635514 Creates some more macros for deduplicating the add_value_to_array type functions.

Addressing PR Feedback commits

• e741afa Removes a debug fprintf
• e617716 Various PR feedback
• 3ff206f Moves impl headers into src directory