This CL implements a streaming decoder which takes the bytes
of a wasm module as an input, potentially split into multiple
chunks, and decodes them into segments. Each segment either
contains the payload of a whole section, or the code of a
single function. The goal is that the streaming decoder is
used for streaming compilation. That's where the interface
comes from, see
(https://cs.chromium.org/chromium/src/v8/include/v8.h?q=OnBytesReceived&sq=package:chromium&l=4060)

Error positions are not reported correctly at the moment. I
plan to do this in a separate CL.

Change-Id: I6e3df6a91945c7baec2dc4f5de2e5f47636083df
Reviewed-on: https://chromium-review.googlesource.com/471350
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45250}