refactor: factor symbol management into a new Symbol.lua module.

* lib/zile/Symbol.lua: New file. Centralize symbol creation and
management here.
* lib/zile/zlisp.lua: Adjust accordingly.
* lib/zmacs/eval.lua: Simplify.  Adjust all callers.
* lib/zz/eval.lua: Likewise.
* lib/zile/zile.mk (dist_pkgdata_DATA): Add lib/zile/Symbol.lua.
* local.mk (dist_doc_DATA): Add doc/classes/zile.Symbol.html.
* doc/config.ld (file): Add lib/zile/Symbol.lua.

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