[wasm] Refactor wasm-serialization to fix alignment

- Alignment of code sections is only relative to start of header. Code
  should be aligned in the buffer.
- Rewrites Reader/Writer classes to make global alignment easier.
- Rewrites the native serialization to simplify things in preparation
  for switch to streaming serialization APIs.

Bug: chromium:719007
Change-Id: I0397bc84a8d009b4fd7f5286bb5abc527bb0db95
Reviewed-on: https://chromium-review.googlesource.com/1027433
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52860}