refactor: simplify std.require, and improve error diagnostics.

Closes #78.
* specs/std_spec.yaml (require): Specify better diagnostics on
failure.
* lib/std/list.lua (compare): Move from here...
* lib/std/base/list.lua (compare): ...to here.
* local.mk (dist_luastdbase_DATA): Add lib/std/base/list.lua.
* lib/std/base.lua: Break dependency on "std.list".
(require): Use base.list.compare directly, and show verbose
diagnostics on failure.
(module_version, version_to_list): Remove; no longer used.
* NEWS: Update.

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