In the process:
- Add ToString to the CodeStubAssembler and use it where appropriate
- Add constant-folding versions of IntPtrAdd/IntPtrSub to simplify code
  in element offset computation, especially for strings.

BUG=chromium:608675
LOG=N

Review-Url: https://codereview.chromium.org/2407813002
Cr-Commit-Position: refs/heads/master@{#40379}