specl: use an unreleased specl with tail-calling objcmp.

Released specl overflows luajit stack when comparing 1000 nested
objects. Pull in the fixed version until Specl 11 is released.
* build-aux/specl: New file, with tail-calling objcmp.
* Makefile (SPECL): Use it.
* lib/sexpr/lisp.lua (ComparableAtom): New root object with fast
comparison metamethods.
(Number, String): Use it.

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