Core.scala API documentation

Remove old Literal object; fold the parseLit into UInt

Update README with Mem(seqRead) -> SeqMem()

Handle IllegalArgumentExcetions that mean a cloneType is needed

When trying to cloneType() on classes that take a builtin (like Int),
we get an IllegalArgumentException instead of a InvocationTargetException.
This change prints a nice error message instead of a stack trace.

Type hierarchy diagram -- looks good

Avoid:

    [error] (run-main-0) java.lang.UnsupportedOperationException: empty.reduceLeft
    java.lang.UnsupportedOperationException: empty.reduceLeft

when invoked via chiselMain() with no Parameters.

Flip Mem/SeqMem argument order; deprecate old ordering

Eliminate outdated parts of readme, add skeleton for new format

Remove unused emulator C++ headers

- Rename Alias to Node to match FIRRTL notion
- Remove poorly-named Immediate and replace root of hierarchy with Arg

Emit read-enables as mux(ren, addr, poison).

Use the companion objects instead.

Now, we emit all I/Os inside a bundle named io.

A mangled name may itself conflict with another name.

No need for Option, since an empty Set can be used instead.

Read enables and read-write ports aren't working yet.

The fix is to propagate the flip to the fields in the bundle.