Add a third parameter to {AsHex} which specifies whether the prefix
"0x" should be printed. Also, add the {AsHexBytes} helper which
outputs the hex number as individual bytes separated by a whitespace.
Also add unit tests for both helpers.

Both helper will be used in an upcoming refactoring of wasm error
messages:
https://chromium-review.googlesource.com/c/565282

R=titzer@chromium.org

Change-Id: I42d5ace9841ffb918cb4d6803b6347229e446097
Reviewed-on: https://chromium-review.googlesource.com/583448
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46859}