We were unnecessarily storing everything as uint32_t, even though many items in
the preparsed scope data can be stored as uint8_t. This CL also adds an
(internal) API which abstracts away the actual data storing, so the backing
store can be made even more efficient (e.g., use only 1-3 bytes for some
uint32_t values, if they fit) without affecting other parts of the code.

BUG=v8:5516,chromium:762492

Change-Id: I7cd4d91dc11f87f8aec9c7584044a6f2a59b73ba
Reviewed-on: https://chromium-review.googlesource.com/684182
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48231}