add new APIs to BitPat (#2076)

* add Y and N to BitPat.

* add ## for BitPat.

* add rawString API.

* use rawString in decoder

* add select and slice to BitPat.

(cherry picked from commit e74b978d)

# Conflicts:
#	src/main/scala/chisel3/util/BitPat.scala
#	src/main/scala/chisel3/util/experimental/decode/EspressoMinimizer.scala
#	src/main/scala/chisel3/util/experimental/decode/QMCMinimizer.scala
#	src/main/scala/chisel3/util/experimental/decode/TruthTable.scala
#	src/test/scala/chiselTests/util/BitPatSpec.scala