A continuation of the work in 59e4b751, this extracts logic around
memory reservation and allocations out of the Deserializer class.

Follow-up work is planned to create a specialized allocator for
builtin deserialization.

Bug: v8:6624
Change-Id: I7081cdc557ab8fb2571aadb816399e136ea2cdbb
Reviewed-on: https://chromium-review.googlesource.com/716036
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48634}