strbuf: derive from `std.object.Object`.

* specs/strbuf_spec.yaml: Describe expected behaviour of strbuf as
an Object.
* specs/specs.mk (specl_SPECS): Add specs/strbuf_spec.yaml.
* std/strbuf.lua (new): Return a new Object, with metamethods and
object methods rolled in.
(M): Return a callable table that forwards module calls to the
`new` function.
* NEWS: Update.

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