Custom allocator for vw library

[rajan-chari]

Allow users of vw library to provide their own allocator.

[jackgerrits]

What are the use cases of this?

Seems like there is a standard way to do this at least:

1. Create a class that conforms with std::allocator if you want a custom allocator. There is even a requirements specification.
2. Allow for custom allocators in all of our library classes where it makes sense through a template parameter defaulted to std::allocator (This is how std::vector does it)

[lokitoth]

We would need to ensure that this allocator is realloc-compatible, or rethink the realloc codepaths.

In particular, we do realloc here:

• https://github.com/VowpalWabbit/vowpal_wabbit/blob/master/vowpalwabbit/parse_args.cc#L221
• https://github.com/VowpalWabbit/vowpal_wabbit/blob/master/vowpalwabbit/parse_regressor.cc#L150
• https://github.com/VowpalWabbit/vowpal_wabbit/blob/master/vowpalwabbit/search.cc#L3180
• https://github.com/VowpalWabbit/vowpal_wabbit/blob/master/vowpalwabbit/v_array.h#L66

There is also a question about passing it in to underlying libraries, e.g. RapidJSON. But that seems to have an allocator interface: https://github.com/Tencent/rapidjson/blob/master/include/rapidjson/allocators.h

A bit of context about realloc and std::allocator: https://stackoverflow.com/questions/6391057/realloc-in-standard-allocator