interval trees: use stdx.allocator / configurable memory alloc strategy

[jblachly]

No description provided.

[jblachly]

moved to https://github.com/blachlylab/intervaltree

[jblachly]

Actually need to work on this in chainfile.

Currently, ChainFile is responsible for allocating the links placed in the Tree. When using AllocatorList!Region!Mallocator, the memory is allocated fine, but when ChainFile goes out of scope the memory is freed, leading to bad accesses in IITree. AVL and SPlay trees seem not affected I think because their insert methods use GC.

I fixed this temporarily by moving back to a manual malloc pool allocator (which never frees since the pool is part of ChainFile and we need it 'til the end of program essentially), but I would like to transition back to std.experimental.allocator in the future.

Probably the correct wayt o do this is to use the thread global theAllocator
https://dlang.org/phobos/std_experimental_allocator.html