Lazy .fir Emission (backport #2083) (#2084)

* Remove chisel3's own firrtl Emitter, use firrtl Serializer

This will be slightly slower as it involves converting from Chisel
modules to FIRRTL modules before turning them into Strings. This cost is
somewhat mitigated by doing that conversion lazily such that we never
materialize the entire firrtl Circuit in memory, only 1 module at a
time.

(cherry picked from commit 73bd4ee6)

# Conflicts:
#	src/main/scala/chisel3/internal/firrtl/Emitter.scala
#	src/test/scala/chiselTests/PrintableSpec.scala
#	src/test/scala/chiselTests/VecLiteralSpec.scala
#	src/test/scala/chiselTests/experimental/verification/VerificationSpec.scala

* Emit .fir lazily, overcomes JVM 2 GiB String limit

(cherry picked from commit d9c30ea0)

# Conflicts:
#	src/main/scala/chisel3/internal/firrtl/Emitter.scala

* Use BufferedCustomFileEmission in CircuitSerializationAnnotation

(cherry picked from commit a6eb2ad8

)

* Resolve backport conflicts

* Waive binary compatibility issues

Co-authored-by: Jack Koenig <koenig@sifive.com>