Equality hypothesis context rules

Each equality expression appearing in ite or implication will populate the equality hypothesis context used when optimizing
- then/else clause of an `ite`
- the conclusion of the `implication`.

Entries in the equality hypothesis context may correspond to one of the followings:
- v := e (when e is not a free variable)
- e1 := C x1 ... xn
- v1 := v2

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**Transferred from:** input-output-hk/sc-fvt#415

No file, test, or entry point is named. Start by locating the equality hypothesis context and its optimization callers; done means the stated equality forms populate the context for ite branches and implication conclusions, with coverage for those cases.

Written by the indexing model from the issue text.