This adds partial support of exports to the runtime system and
to the interpreter. It introduces a new HeapObject JSModule that
maps each of the module's export names to a Cell containing the
exported value.

Several aspects of this implementation are subject to change in
follow-up CLs.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2302783002
Cr-Commit-Position: refs/heads/master@{#39341}