refactor: put api and helper functions in sections.

* lib/std/array.lua, lib/std/container.lua, lib/std/io.lua,
lib/std/optparse.lua, lib/std/package.lua, lib/std/set.lua,
lib/std/string.lua, lib/std/table.lua, lib/std/tree.lua: To make
it clear that user-facing code ("api functions") need argument
checking, but internal code ("helper functions") does not,
explicitly separate those kinds of functions, and add some
block headers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>