Cleanup apply_deltas (#1464)

Next step in cleanup of pattern-matcher:

* Each rewrite-transformation is applied immediately when it is
identified (since the IR now allows us to do this).
* Eliminate the extra checks to avoid overlapping matches (because of
above)
* Use a structured representation of the delta
* Enable function constructing the replacement to return None (failure
of rewrite)
* Cleanup MatchResult
* Other naming cleanup

TODO:
* Consider extending delta representation to capture old/new
output-values

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Co-authored-by: Ti-Tai Wang <[email protected]>
Co-authored-by: Justin Chu <[email protected]>