1) The code table never grows, so store it in a heap-allocated byte
   array instead of an std::vector.
2) Rename {functions_count} to {num_functions} for consistency with
   {num_imported_functions} and occurences in other data structures.

R=mstarzinger@chromium.org

Bug: v8:7754
Change-Id: Id9d66545ed7aa675d663dad5936a9ef6d44ace7d
Reviewed-on: https://chromium-review.googlesource.com/1066014
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53488}