Move Bits, Data, and BitPat to chiselFrontend/src/main/scala/chisel3

 and deal with the subsequent fallout.

Move Aggregate, Clock, Mem, Printf, Reg

Move almost all chisel3.core definitions to chisel3 or chisel3.experimental

Revive core package object to provide aliases for moved definitions.

Cleanup package definitions; eliminate ambiguous implicits

Move more definitions to experimental.
Extract BaseModule, DataMirror, ExtModule, IO into their own files.

Add EnumType definition to core package.
Update deprecated messages to refer to correct object

Move FixedPoint into the experimental package (but keep it in Bits.scala).

Add missing implicits to core/package - compatibility

Cleanup: update ScalaDoc references; remove unused imports

Use common 3.2 version in deprecation warning

Move Binding from core to internal.