First stab at strongly typing the wasm compiled module FixedArray. The goal
is to improve maintainability of the data structures living on the JS heap.

My goal is to do so just for the first level, since we plan to eventually
avoid copying the metadata bits that are currently copied from the decoded
structures (export/import metadata, etc).

Subsequent CLs will try and consolidate internal functions working off the
compiled module as members, and evaluate what the actual interface with the
rest of the world of this type should be - we may be able to completely
move it in the cc file, for instance.

BUG=

Review-Url: https://codereview.chromium.org/2340623003
Cr-Commit-Position: refs/heads/master@{#39890}