refactor: remove deprecated methods.

* lib/std/base.lua (new, metatable): Remove.
(concat, M): Simplify accordingly.
* lib/std/functional.lua: No need to require std.base.
* lib/std/set.lua (_functions): Move into object declaration,
removing `new`.
* lib/std/strbuf.lua (new): Remove.
* lib/std/tree.lua (new): Remove.
* NEWS: Update.

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