In many places in WasmModule and contained structs we store references
into the wire bytes as pairs of offset and length.
This CL introduces a WireBytesRef struct which encapsulates these two
connected fields. This makes it easier to pass them and assign them as
one unit.

R=ahaas@chromium.org, mtrofin@chromium.org
BUG=v8:6474

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I4f2a40d848a51dc6f6f599f9253c3c6ed6e51627
Reviewed-on: https://chromium-review.googlesource.com/530687
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45859}