Improve implementation of 'forward-thing' using custom providers

Improve implementation of 'forward-thing' using custom providers

Now, call all the custom providers for each step, using the provider
that moves point the smallest non-zero amount.  This allows multiple
providers for a given "thing" to work nicely together.

* lisp/thingatpt.el (forward-thing-provider-alist): Update docstring.
(forward-thing): New implementation to call each provider N times.
(forward-thing-for-text-property): Take BACKWARD instead of N.  Update
callers.

* test/lisp/thingatpt-tests.el (thing-at-point-providers)
(forward-thing-providers): Add more checks.
(consecutive-things-at-point): New test.