debug: argcheck accepts objects as valid table type arguments.

Closes #84.
Really, table is the base type of object (or strictly speaking,
container), and its almost always desirable to be able to pass
objects to functions that operate on tables.
* specs/debug_spec.yaml (argcheck): Specify this behaviour when
an object argument is given where a table is required.
* lib/std/debug.lua (argcheck): Accept any object argument
where a table parameter is expected.

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