No major functional changes in this CL, mostly it moves code around
to make follow-up CLs less messy.

 - Document Code layout.
 - New concepts: 'body' and 'metadata' areas of Code objects. The
   metadata area contains metadata tables, the body area includes
   both instructions and metadata (this is currently the 'instructions'
   area). Add accessors for these new areas.
 - An interesting detail: embedded builtins will have non-adjacent
   instruction and metadata areas, thus a concept of 'body' doesn't
   make sense there.
 - Also add raw_instruction_X_future accessors; these are used where
   we are actually interested in the instructions range, not the entire
   body. In a follow-up, current raw_instruction_X accessors will be
   replaced by raw_body_X, and raw_instruction_X_future by
   raw_instruction_X.

Bug: v8:11036
Change-Id: I1d85146b652e0c097c3602d4db1862d5d3898a7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491023
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70701}