table: deprecate totable.

Closes #74.
* specs/container_spec.yaml (tablification): Remove specs.
* specs/object_spec.yaml (copy): Use this new function instead of
`totable`.
* specs/object_spec.yaml, specs/set_spec.yaml,
specs/strbuf_spec.yaml, specs/tree_spec.yaml (__totable): Remove.
* specs/table_spec.yaml (totable): Deprecate this function.
* specs/spec_helper.lua (totable): Remove.
* lib/std/table.lua (totable): Likewise.
* lib/std/base.lua (totable): Remove implementation.
* lib/std.lua.in (barrel): Remove "table.totable".
* lib/std/container.lua (__totable): Remove.
(__pairs): Return elements in order, omitting private elements.
(__tostring): Use it to concatenate elements in order.
(__call, instantiate, mapfields): Iterate with `next` to fetch
raw elements, rather than ordered object __pairs elements.
* lib/std/object (__totable): Remove reference.
* lib/std/set.lua (__totable): Remove.
(__tostring): Fetch keys for display using std.pairs.
* lib/std/string.lua (pickle): Likewise.
* NEWS: Update.

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