toBigInt should round identically for Double and BigDecimal (#3921)

---------

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@gmail.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>
(cherry picked from commit b9f57032)

# Conflicts:
#	core/src/main/scala/chisel3/Num.scala
#	src/test/scala/chiselTests/LiteralExtractorSpec.scala