Extend secondary allocation support

[mjp41]

Here are some thoughts for things we can do

• [ ] Add support for refilling the free list to a particular size, e.g. refill(LocalCache&, sizeclass, size_t& request). refill will add at most request entries to the freelist, removes the count of added entries from request.
• [ ] Using modified refill adapt GWP-Asan sampling to accurately reflect the sampling request from the user.
• [ ] Add documentation for building with GWP-Asan.
• [ ] Automate downloading GWP-Asan.
• [ ] Provide support for Windows using detours, and a secondary allocator of the original Windows Heap.
• [ ] Build sampling profiler using secondary allocator.
• [ ] Add C++ Concept
• [x] Enable out of tree override of secondary allocator.

This is a bit of a brain dump of things I think this feature enables. My thought is to try to get a reasonable number of these together, and release as 0.8.0.

@SchrodingerZhu, or anyone else, please feel free to add your own ideas on use cases.

[SchrodingerZhu]

It would be also good to make a concept with static_assert for c++20.