Extract Zlib/Brotli custom allocator

[npaun]

• Zlib and Brotli use malloc and free by default to manage memory. If they leak, it's an upstream bug.
• However, they also allow for custom allocators to be passed.
• This PR extracts our custom allocator functions AllocForZlib, AllocForBrotli, FreeForZlib into a new class called Allocator.
• Allocator is now used for both the streaming and one-shot implementations.
• I dropped the stuffing of size into the allocated regions because the backing kj::heapArrays are already sized objects.
• At present, the information collected by allocator is not used. However, we could block excessively large allocations, and report allocations to V8 in the future.
• Our current implementation is a hash map of pointers, but what we'd really like is a kind of arena allocator, in my opinion at least.

Closes #2678

[jasnell]

Let's have @anonrig review also