[runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

[runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

This adds a new ToString runtime function and a fast-path ToStringStub
(which is just a simple dispatcher for existing functionality), and also
implements %_ToName using the ToStringStub.

R=mstarzinger@chromium.org, yangguo@chromium.org
BUG=v8:4307
LOG=n

Review URL: https://codereview.chromium.org/1319973007

Cr-Commit-Position: refs/heads/master@{#30442}