Replace conduit handler function pointers with std::function

[Arlie-Capps]

Propose to change set_memcpy_handler(), set_memset_handler(), and AllocManager and all associated interfaces to use std::function. This would require no change to existing code, since existing code assigns or passes a function pointer and function pointers are assignable to std::function. This change would add valuable functionality, since user code would then be able to set up lambdas or other types of callable as the copy, set, alloc, and free operations.