specs: more comprehensive specs for std.string.split.

* specs/string_spec.yaml (split): Add a lot more specs.
Empty separator causes an infinite loop with the current
implementation.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
string: simplify and speed up `std.string.split`.

* lib/std/string.lua (split): It's a bit more code, but it is
much easier to understand, and it's a lot faster now!
* specs/string_spec.yaml (split): Uncomment pending infinite loop
fix.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
string: split now splits on consecutive whitespace by default.

* lib/std/string.lua (split): Improve LDocs.
Remove @todo. We now have whitespace default split pattern unless
otherwise specified, just like Python.  I didn't consider Perl,
because my brain starts to haemorrhage whenever I do that. :-p
* specs/string_spec.yaml (split): Remove spec for string `sep`
argument now that it is optional.
* NEWS: Update.

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