ir: add faster serializer (#1694)

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>

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