container: discard unmapped positional parameters.

Fix #35.
* lib/std/container.lua (mapfields): When `map` argument is
given, discard unmapped positional parameters.
(metatable._init): Remove to imply a `nil` value (copy all
fields), because `{}` now discards all positional parameters.
(metatable.__call): Reverse type check of `_init`, so that
mapfields is still called when _init is `nil`.
* specs/object_spec.yaml (std.object): Remove pending call from
newly passing examples.
* NEWS: Update.

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