Simplify Enum API (#385)

Get rid of some cruft exposed in #373

This also allows Bits.fromtInt(...) to be removed. Yay!

All old APIs (with some new restrictions, rocket still works fine) are preserved without deprecation in Chisel._, aside from the non-compile-time-checkable Map[] enum constructor which probably should have been deprecated during chisel2. The Map[] enums have been removed from chisel3._ without deprecation.

The new restriction is that nodeType (legacy API) may only be of UInt type with unspecified width. Note that Bits() creates a UInt, and if you can't control the enum values, it makes little sense to specify a bitwidth.