matchers: equal matcher now understands std.objects.

* specs/matchers_spec.yaml: Specify desired behaviour when
comparing objects with the equal matcher.
* lib/specl/matchers.lua (objcmp): Compare the contents of
std.object derived types.
* docs/specl.md (equal): Update.
* NEWS: Update.

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