[feat]: add custom allocator

[broccoliSpicy]

This PR tries to add a class custom_allocator_gt so we can pass different allocation/free function pointers to usearch.

lantern issue #147

for discussion only, just a draft to set the direction.

todo: how much performance benefits we can get if we use a aligned allocator(palloc_aligned())

[broccoliSpicy]

these are just operators to conform c++'s requirement for an allocator

[broccoliSpicy]

Hey @Ngalstyan4, would you be a lamb and check out this PR for me?

[Ngalstyan4]

Hi @broccoliSpicy , thanks for the contribution!
Could you share the intended API for passing the allocation functions from C as well? And whether/how postgres's palloc could fit into the allocator interface.

It might also be useful to look at pgv8 postgres extension for how to deal with cpp allocations in a postgres context

Thanks again for your contributions!

[broccoliSpicy]

passing the allocation functions from C

as for checking pgv8, I am sorry that I am having final projects and final exams this week, but I will definitely check it out in next week.

[broccoliSpicy]

I checked pgv8, I think their cpp allocator is more like a buffer pool, and I couldn't see their cpp allocator's relevance with postgres's palloc, it seems their cpp allocator is just using standard c malloc function and couldn't be customized.