Implement the logic for StringIterator.prototype.next in the JSBuiltinReducer in order to allow inlining when the receiver is a JS_STRING_ITERATOR_TYPE map, built ontop of the SimplifiedOperators StringCharCodeAt and the newly added StringFromCodePoint.

Also introduces a new StringFromCodePoint simplified op which may be useful for other String builtins, such as String.fromCodePoint()

BUG=v8:5388
R=bmeurer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2373983004
Cr-Commit-Position: refs/heads/master@{#39994}