Consider shipping a managed allocator

### Describe the enhancement requested

A managed allocator would not conform to the Arrow specification in that its buffers wouldn't be 64-byte aligned. But in e.g. a purely-managed service environment where the buffers are being assembled to ship out on a wire, the alignment shouldn't really matter and the managed allocator might be substantially faster, at least for small allocations. (Impact on LOH would need to be understood.)