Source Mapper Improvements: Small Fixes

## Problem

There were a few issues that were not resolved at the time of merging #650 .

## Solution - Implement the following small improvements

- [x] (#650 already addressed) Clarify the legend for `_PyTealMapper.annotate()` (cf: https://github.com/tzaffi/pyteal/blob/f2a945a8eeb396e9c544b62cd3d1a6383bfc9bff/pyteal/compiler/sourcemap.py#L844)
- [x] Rename confusing `StackFrame._frame_info_is_right_before_core()` (renamed in #650)
- [x] Prune unused methods in the `StackFrame - PyTealFrame - TealMapItem` hierarchy (#689)
- [x] Final status checks (ensure all changes are actually necessary) (#689)
- [x] which of the usages of `._sframes_container` are actually necessary. To see all usages: [f96ef86](https://github.com/algorand/pyteal/pull/650/commits/f96ef8657124466884974501137764a3f7c0a266)
- [x] audit the following test files and either remove, unskip, or provide clear commentary as to why the skipped test needs to stay in (#689 unified using `STABLE_SLOT_GENERATION: bool` constants currently `False):
- [x] tests/integration/sourcemap_monkey_integ_test.py
- [x] tests/unit/sourcemap_constructs311_test.py
- [x] tests/unit/sourcemap_constructs_allpy_test.py
- [x] tests/unit/sourcemap_monkey_unit_test.py
- [x] tests/unit/sourcemap_test.py
- [x] At the end of #676 pay close attention to `_PyTealSourceMap._infer()` and see which lines are still being hit. Probably, there will be some situations that are no longer needed. (#689 tightened this up)
- [ ] Refactor `NatalStackFrame` to provide a single frame instead of a list
- [ ] Coverage triage
- [ ] first item to notice: we could do better on subroutines such as the recursive fibonacci example:
- [ ] with scratch slots: https://github.com/algorand/pyteal/blob/6a49be7b4c74abe6fa08493bdb8e5585dabf0bca/tests/unit/sourcemap_constructs311_test.py#L1901
- [ ] with frame pointers: https://github.com/algorand/pyteal/blob/6a49be7b4c74abe6fa08493bdb8e5585dabf0bca/tests/unit/sourcemap_constructs311_test.py#L1955
- [ ] Pull in updated version of `mjpieters`'s **SourceMap** to get faster R3SourceMap (cf: https://github.com/tzaffi/pyteal/blob/f2a945a8eeb396e9c544b62cd3d1a6383bfc9bff/pyteal/compiler/sourcemap.py#L75)
- [ ] Investigate the feasibility of removing the `_sframes_container` property from `class TealComponent` and whenever this is needed, simply overwrite either `self.expr.stack_frames` or `self._stack_frames`. (partially removed from much of hierarchy in #689)
- [ ] If feasible, implement it
- [ ] Also consider having a new method `NatalStackFrame.spawn()` that keeps the original `NatalStackFrame` as a child.
- [ ] It's likely that `_PyTealSourceMapper` will never be exposed to the end-user. In that case, the logic for caching build items is overly complex. This should be simplified at the end or shortly after #676
- [ ] Less clunky workflow and artifact access in `_PyTealSourceMapper` (cf: https://github.com/tzaffi/pyteal/blob/f2a945a8eeb396e9c544b62cd3d1a6383bfc9bff/pyteal/compiler/sourcemap.py#L730)

## Dependencies

None

## Urgency

Medium - while the urgency of any improvement individually is Low, addressing at least a few of these issues in the short term will improve the feature's usefuleness.