Enhance SlabStore documentation with security considerations and add fallible try_new method (#107)

- Expanded module documentation to include a comprehensive security section, detailing risks associated with the `FxHashMap` hasher and the implications of using `EntryId`s across removal boundaries.
- Introduced `try_new` method to provide a fallible variant of `new`, allowing for explicit handling of allocator failures, particularly when capacity may originate from untrusted input.
- Updated existing methods to improve clarity on panic conditions and internal invariants, ensuring better understanding of potential pitfalls in usage.

These changes improve the robustness and security of the `SlabStore`, aligning with best practices for handling untrusted input and enhancing overall documentation clarity.

Author: TKorr