dedup: use structural sha256 hash instead of agnostify and serialize (#1731)

* benchmark: add hot.DedupBenchmark

* dedup: use structural md5 hash instead of agnostify and serialize

* StructuralHash: generate PrimOp LUT

* StructuralHash: change MessageDigestHasher to not be a case class

* StructuralHash: we want Blocks and EmptyStmt to be ignored

* StructuralHash: use SHA-256 instead of MD5

* StructuralHash: clarify extmodule port name agnistification

* StructuralHash: hash the name of width vars instead of trying to agnostify

This should be in line with the old Dedup behavior.
The prior use of n(..) was incorrect since the namespace
of these vars is different from the normal module scope namespace.

* StructuralHash: address Schuyler's review comments

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>