std: remove __ipairs support in favour of 1..#t iteration.

* specs/std_spec.yaml (ipairs, ireverse, ripairs): Adjust
specifications to verify treatment of __len metamethod, and
ignore __ipairs metamethod.
* lib/base.lua (ipairs): Using __len if available, or # operator
otherwise, iterate over elments 1..#t.
(unwrap__ipairs): Remove.  Simplifications above make this
function superfluous.
(ripairs, ireverse): Adjust accordingly.
* lib/std/vector.lua (core_metatable.__ipairs)
(alien_functions.__ipairs): Remove.
* lib/std.lua.in (ipairs, ireverse, ripairs): Update LDocs.
* NEWS: Update.

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