Cleanup and simplify internal logic

[jeremykubica]

The goal of this PR is to cleanup and simplify the internal logic for sampling so it is easier to understand and debug. To this end, it:

• Adds a __str__() function to GraphState that produces a human readable debugging string.
• Replaces the nodes' hash value with the nodes' (readable) string in GraphState. This will add slight overhead in indexing (to hash the string), but greatly increase testability and debug-ability.
• Switch the PyTree to use the same dictionary structure as GraphState (a double index of [node_name][var_name]). This allows:
  • Removing the now unused full_name attribute from ParameterSource
  • Removing a lot of specialized logic that was needed to handle given_args. Instead the given_args are merged directly into the current GraphState as fixed parameters.

[github-actions[bot]]

Before [9fcdf6dd]	After [68e0241c]	Ratio	Benchmark (Parameter)
6.95±0.06ms	6.87±0.03ms	0.99	benchmarks.time_chained_evaluate

Click here to view all benchmarks.