refactor: reduce module dependencies of std.debug.

* lib/std/debug.lua: Remove std.io dependency by using core
file:write instead of std.io.writelines.
Remove std.list dependency by using lighter functional.map instead
of list.map.
Remove unused std.object dependency.
(tabify): Instead of a deeply nested in-situ call to list.map
and others, functionally compose an equivalent and use that to
simplify.

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