v23: Replace RawBytesTarget hacks with vdl.Targeter

The previous RawBytesTarget{Func,Hack} hacks were meant to
support making and filling a vdl.Target from vom.RawBytes
directly.

This CL makes this mechanism more general-purpose and slightly
less hacky.  We introduce a new vdl.Targeter interface with
{Make,Fill}VDLTarget methods, which make and fill a vdl.Target
respectively.  These methods are implemented by vom.RawBytes, and
in the future will be implemented by user-defined types via vdl
code generation.

The HackGetRv() method hasn't been removed yet; that will be
removed in a subsequent change.

MultiPart: 1/2

Change-Id: I00742c91eb97d67ba19895f766cef74763018bcc
8 files changed