base: provide better errors from exported object methods.

* specs/base_spec.yaml (export): Specify behaviour of exported
object methods.
* lib/std/base.lua (export): Use separator ':' between module name
and method name when dealing with methods, as opposed to '.' when
dealing with functions.
Count method arguments starting at '0' for self in error messages.
* build-aux/sanity-cfg.mk (sc_error_message_uppercase): Add
specs/base_spec.yaml.

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