refactor: decouple std.lua from other modules.

* lib/std/lua.lua (wrapiterator): Move from here...
* lib/std/base.lua (wrapiterator): ...to here.
(ielems): Non-argchecked implementation.
* lib/std/lua.lua (ielems): Use it.
* lib/std/debug.lua, lib/std/list.lua, lib/std/set.lua,
lib/std/tree.lua: Use base.ielems internally.
* lib/std/functional.lua (case, eval, lambda, memoize): Load
std.lua on demand when these functions are called rather than
depending on it at require time.
* lib/std/table.lua: Remove unused std.lua requirement.

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