Use an unordered_map<Module, unordered_set<String>> to keep track
of visited Module/ExportName pairs during ResolveExport.

This required adding a Hash() method to Module, which is accomplished
by allocating a Symbol and storing it in the SharedFunctionInfo::name
slot, then delegating the hash to that Symbol.

Also added a helper method Module::shared() to easily get ahold of
the SharedFunctionInfo and call it in the appropriate places instead
of re-doing the ternary operator.

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2367623004
Cr-Commit-Position: refs/heads/master@{#39743}