Add a preconditions package that supports writing precondtions

checks and offers a handful of checks for common needs.

TODO: possibly add fine-grained whitelist and/or blacklist
support akin to logging configurtions that allow just turning
on checks for a critical set of user-facing functions or else
targeting a performance critical set of function for disabling
of checks.

https://rbcommons.com/s/twitter/r/118/