refactor: revert to std.set over zile.Set.

Recent std.container based stdlib sets are now just as fast as,
and more featureful than the custom zile Set objects.
* lib/zile/Set.lua: Remove.
* lib/zile/zile.mk (dist_pkgdata_DATA): Remove lib/zile/Set.lua.
* lib/zile/Symbol.lua, lib/zile/zlisp.lua: Adjust `require`s.

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