This CL refactors allocation & reservation logic into a new
DefaultSerializerAllocator class.  In upcoming work, this will be
further extended by a custom allocator for builtin serialization.

Additionally, this cleans up a bunch of cosmetics (encapsulation and
other nits).

Bug: v8:6624
Change-Id: Ibcf12a525c8fcb26d9c16b7a12fd598c37a0e10a
Reviewed-on: https://chromium-review.googlesource.com/650357Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48077}