Implement a fast buffer allocator for Vector, HashTable and StringBuilder

Implement a fast buffer allocator for Vector, HashTable and StringBuilder

PartitionAlloc is optimized for fixed-sized object allocations. If we use PartitionAlloc for expandable buffers such as Vector, HashTable and StringBuilder, we will end up with reallocating & copying the underlying buffers almost every time they are expanded/shrunk.

We need an allocator that is optimized for expandable/shrinkable buffers (i.e., we need an allocator that can expand/shrink buffers without reallocating & copying the buffers.) With that motivation, this CL exports Oilpan's buffer allocator to non-Oilpan Vectors, HashTables and StringBuilders.

This CL is still a WIP. Not ready for review.

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