Support public/1 directive, add public_clauses flag (#372)

[guregu]

See: #372. Applied feedback from @UWN.

Changes:

• Support public/1 directive
• Add public_clauses flag that bypasses the public-private check when on (off by default)
  • Feel free to name this something else
• Add predicate_property/2 for publicness
• Undo strict_iso stuff for clause/2 that was added in Support clause/2,3 for non-dynamic clauses #373

Internal stuff:

• Added is_exported bit for predicates
• Use is_exported instead of is_public for module exporting stuff
• Remove a singleton var from the tests I added 🙈
• Add a test for public/1 directive
• I left module.make_public alone as I wasn't sure exactly what it was for.

[guregu]

Closing in favor of a system predicate as per #372 (comment)