Allocator Interface

[MrBurmark]

Add Allocator Interface that exposes more options and state to users.

Refactor our existing internal allocators to use an Allocator interface.
Add ability to set allocators with user defined Allocators.
Add ability to get individual allocators or all allocators in use.
Add ability to query allocator sizes.

• This PR is a refactoring, bugfix, and feature
• It does the following (modify list as needed):
  • Refactors basic_mempool into AllocatorPool
  • Fixes Can not query RAJA internal memory pools size #1023
  • Adds ability to query allocators sizes and change allocators

Design review

On some date, we'll review this PR. We may discuss the design ideas:

1. Make allocators more standard
2. Make allocators replaceable by users
3. Make internal pool sizes queryable by users
4. Use a std allocator interface
5. Multiple Resource Safety
6. Multiple Device Support.

This PR implements 1. 2. and 3. It leaves out 4. 5. 6. for the following reasons

• 4 Not sure how to make sizes user queryable
• 5, 6 Keep Scope manageable for next release

[artv3]

This looks great! Can we get example of how to use under the example folder?

[trws]

When you say you want the sizes to be user queryable, do you mean something like a pool size or what? Your fourth point should be possible, but querying size will likely either work only for a pool-like allocator or give a non-useful answer (for a lot of reasons, have a look at the docs on jemalloc usage functions for details).

[MrBurmark]

I agree, I think I'll drop the ability to query the size. Then expose something for only the internal RAJA pool allocators that supports querying the size in a similar manner to umpire pools. The user will be responsible for tracking the sizes of any user-provided allocators.

[trws]

That makes sense. Especially if you document the concept used to query the size so a user can implement it if they want to.