refactor: break std.debug dependency on std.functional and std.string.

* specs/debug.spec (say): Specify usage of std.tostring.
* lib/std/debug.lua (tabify): Remove.
(say) Manually unroll functional compose sequence, formerly
knows as tabify.
(trace): Manually unroll and consequently remove string.rep call.

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