Imports and exports in 0xC can be much more than functions, including
tables, memories, and globals. This CL refactors the underlying
organization of imports and exports to support these new import types.

BUG=

Committed: https://crrev.com/599f8a83420346d9cba5ff97bd2a7520468207b6
Review-Url: https://codereview.chromium.org/2390113003
Cr-Original-Commit-Position: refs/heads/master@{#40033}
Cr-Commit-Position: refs/heads/master@{#40050}