Instead of passing and storing uint8_t* and size_t, we now more often
store a Vector or OwnedVector, which encapsulates these two fields.
This provides some more DCHECKs which happen automatically when getting
a subvector into the buffer.

Drive-by: More refactorings and removals of unneeded or rarely used helper
methods.

R=ahaas@chromium.org

Bug: v8:8238
Change-Id: I1b24f90f1517fc50360854f46c2d001075544858
Reviewed-on: https://chromium-review.googlesource.com/c/1317817
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57263}