Sets the code up so it'll be easier to have section names as strings instead of
hard-coded numbers. Using strings will require synchronizing with sexpr-wasm.

Mostly NFC (besides now skipping *all* unknown sections).

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1743773002

Cr-Commit-Position: refs/heads/master@{#34464}