Add new convenience packages.

[make-github-pseudonymous-again]

• js-persistent-array (can be derived from seq).
• js-persistent-object (can be derived from ordseq).
• js-persistent-set (can be derived from ordseq, but not really, because we do not have access to object addresses, or hash...).
• js-persistent-map (can be derived from ordseq, but not really, because we do not have access to object addresses, or hash...).

Would probably be good to have Object cast all their keys to strings. For Set and Map, we should only accept immutables as keys: primitives like Numbers, Strings, Dates, RegEx, BigInts, and other Persistent Data Structures.

[make-github-pseudonymous-again]

For set operations see https://arxiv.org/abs/1301.3388