This introduces a new heap object ModuleInfo, which is to ModuleDescriptor
what ScopeInfo is to Scope.  When deserializing a scope info that contains
a module info, we deserialize the module info into a module descriptor and
put it into the synthesized module scope.

Currently, module infos only store exports.

R=adamk@chromium.org
BUG=v8:1569

Review-Url: https://codereview.chromium.org/2277253003
Cr-Commit-Position: refs/heads/master@{#39049}