Refactor/cleanup in `diff-containers` package

[jorisdral]

The prototype code in the diff-containers package has recently been moved to this repository from ouroboros-network/ouroboros-consensus. The code currently still has some artifacts that are specific to ouroboros-consensus, and there is some redundant code that is left over from prototyping efforts. Before releasing the package diff-containers to CHaP, we should clean up the code to remove any of these artifacts and redundant code.

Linked PRs:

• diff-containers: Remove Values and Keys newtypes, use Map and Set instead #9
• diff-containers: Simplify the DiffHistory and NEDiffHistory types #10
• diff-containers: Remove Unsafe* versions of DiffEntry constructors #11
• diff-containers: Remove Act datatype and scrutinous application of diffs #12
• diff-containers: Hide unsafe definitions behind a safe interface #14
• diff-containers: Bring back unsafe constructors #15
• diff-containers: Test preconditions, invariants and postconditions for positivity and normality. #16

[jorisdral]

Resolved: all PRs are merged.