sysutils/bkt: Add new port

bkt (pronounced bucket) is a subprocess caching utility written in Rust,
inspired by bash-cache.
Wrapping expensive process invocations with bkt allows callers to reuse recent
invocations without complicating their application logic. This can be useful in
shell prompts, interactive applications such as fzf, and long-running programs
that poll other processes.

When bkt is passed a command it hasn't seen before (or recently) it executes
the command synchronously and caches its stdout, stderr, and exit code.
Calling bkt again with the same command reads the data from the cache and
outputs it as if the command had been run again.