Implement dynamic arenas

[historic_bruno]

Currently we have a fixed size arena allocator, useful when we have lots of small allocations of a fixed count, but not so useful when that count is unknown. We should implement a dynamic arena that allocates fixed size chunks, and use it for the binary serializer's backrefs map (see #1842).

[historic_bruno]

WIP dynamic arena

[ben]

In 13916:

Adds DynamicArena allocator that grows by fixed chunk size, fixes #2142.
Changes fixed size arenas to new dynamic arenas with reasonable chunk sizes (may require tuning), refs #1842