Modify variant registry to have UnaryOp and BinaryOp registrations. Speed up registry lookup.

* Op type is described as an enum (separate enums for unary and binary ops).
* Modified ZerosLike registrations to unary registrations with ZEROS_LIKE enum.
* Added Add(a,b) registrations as binary registrations with ADD enum.
* AddN op uses ADD BinaryOp registrations and ZerosLike op modified to use
  ZEROS_LIKE UnaryOp registrations.
* Modified the registry tables' keys from string type to StringPiece type.
  The reduced copying should speed up registry lookups by ops.  Required creating
  a backing store for device and type_name strings passed in at registration.

PiperOrigin-RevId: 168020449