This makes use of the newly introduced cell indices to speed up variable
accesses. Imports and local exports are now directly stored in (separate)
arrays. In the future, we may merge the two arrays into a single one, or
even into the module context.

This CL also replaces the LoadImport and LoadExport runtime functions with
a single LoadVariable taking a variable index as argument (rather than a
name).

BUG=v8:1569

Review-Url: https://codereview.chromium.org/2465283004
Cr-Commit-Position: refs/heads/master@{#40808}