* Add a sibling interface to InterpreterAssembler called
  CodeStubAssembler which provides a wrapper around the
  RawMachineAssembler and is intented to make it easy to build
  efficient cross-platform code stubs. Much of the implementation
  of CodeStubAssembler is shamelessly stolen from the
  InterpreterAssembler, and the idea is to eventually merge the
  two interfaces somehow, probably moving the
  InterpreterAssembler interface over to use the
  CodeStubAssembler. Short-term, however, the two interfaces
  shall remain decoupled to increase our velocity developing the
  two systems in parallel.
* Implement the StringLength stub in TurboFan with the new
  CodeStubAssembler. Replace and remove the old Hydrogen-stub
  version.
* Remove a whole slew of machinery to support JavaScript-style
  code stub generation, since it ultimately proved unwieldy,
  brittle and baroque. This cleanup includes removing the shared
  code stub context, several example stubs and a tangle of build
  file changes.

BUG=v8:4587
LOG=n

Review URL: https://codereview.chromium.org/1475953002

Cr-Commit-Position: refs/heads/master@{#32508}