Adopt new convention for marking CLI fns.

Some fns are intended for execution from the CLI (well, currently all of them
are, but in the future maybe some of them will be internal-only). Mark fns that
are callable from the CLI with the metadata ^{:cli true}. Then check to ensure
that a fn named on the CLI is so marked. Throw an exception if it is not.
Define a factorial seq, and two convenience fns.

The convenience fns let you access the seq from the CLI.
Add directions to access f3-seq from the CLI.
Beef up our unit testing to handle the new f3-seq.