We currently have a fixed limit of 256 characters for error messages
generated in the decoder. However, we sometimes embed names in it,
which makes it easy to generate a crash by using long names (e.g. for
exports) in invalid wasm modules.
This CL fixes this by switching to a stream based interface, allowing
to pass arbitrary objects to be printed. With this interface, we can
easily limit the length of output later.

R=titzer@chromium.org

Bug: chromium:740023
Change-Id: I2848c31c63a015157e2a3a9458b54e523060cd69
Reviewed-on: https://chromium-review.googlesource.com/565282Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46860}