Make crate visible methods of `OverlayedChanges` public (#2597)

# Description

- What does this PR do?
This PR make some methods of `OverlayedChanges` public which were
previously only visible in same crate.
- Why are these changes needed?
Since, some methods of the `OverlayedChanges` only have crate level
visibility, which makes `OverlayedChanges` somewhat unusable outside the
crate to create custom implementation of `Externalities`. We make those
method public to enable `OverlayedChanges` to remedy this.
- How were these changes implemented and what do they affect?
Changes are implemented by replacing crate visibility to public
visibility of 4 functions.


# Checklist

- [x] My PR includes a detailed description as outlined in the
"Description" section above
- [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process)
of this project (at minimum one label for `T`
  required)
- [ ] I have made corresponding changes to the documentation (if
applicable)
- [ ] I have added tests that prove my fix is effective or that my
feature works (if applicable)

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Co-authored-by: Bastian Köcher <[email protected]>