StableBTreeMap Implementation from Sardariuss

[kentosugama]

Work by @sardariuss AKA Tanguy on a clone of StableBTreeMap implementation in rust. Related to bounty by ICDevs.

See the original work here: https://github.com/sardariuss/MotokoStableBTree

The goal here is to provide a layer of abstraction over stable memory in the form of a map.

TODOs / questions left:

• Code review
• Review API and unify it with other data structures in the library
• Should the src/StableBTree directory be flattened/refactored into a single module?
• Integrate test/stableBTreeTest tests with the rest of the base library tests
• Extend utility functions (in a later PR)
• Memory manager reportedly a performance bottleneck. Need to investigate
• Need to consider how this interacts with future work on a page allocator to avoid breaking changes in the future.
• Documentation and examples