zssp has been moved into it's own crate.

zssp has been cut up into several files, only the new zssp.rs file contains the critical security path..

zssp was inconsistently using a local/remote vs alice/bob naming convention, all names have been switched to a local/remote convention.

Standardized the naming conventions for security variables throughout zssp.

Implemented a safer version of write_all for zssp to use. This has 3 benefits: it completely prevents unknown io errors, making error handling easier and self-documenting; it completely prevents src from being truncated in dest, putting in an extra barrier to prevent catastrophic key truncation; and it has slightly less performance overhead than a write_all.