* 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>

This Serializer which is implemented
external to the IR node definition
uses a StringBuilder to achieve about a
1.7x performance improvement when serializing.

Eventually, all implementations of the
`serialize` methd should be replaced with
a call to `Serializer.serialize`.

However, for this PR we keep the old
code in place in order to allow for easy
regression testing with the benchmark JAR
like this:
> java -cp utils/bin/firrtl-benchmark.jar \
  firrtl.benchmark.hot.SerializationBenchmark \
  ~/benchmarks/medium.pb 2 5 test

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

* Batch renames in LowerTypes

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

* Add benchmarking for LowerTypes

Add infrastructure of benchmarking Transforms (in addition to
existing infra for Passes). Also run System.gc between each timed
benchmark to improve stability.

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

* Extend PassBenchmark to benchmark a particular Pass
* Add license note to file