Iterators: support transform() with JDK Function class

[reschke]

Currently:

https://guava.dev/releases/snapshot-jre/api/docs/com/google/common/collect/Iterators.html#transform(java.util.Iterator,com.google.common.base.Function)

...would be good to relax that to

transform(java.util.Iterator,java.util.function.Function)

so it becomes easier to migrate away from Guava Function class.

[cpovirk]

Thanks. We may do this eventually for at least some of our APIs. (previously: #2758)

Some things we'd want to consider:

• We can't do this for our Android flavor until our Function extends the JDK Function there (as it already does in the JRE flavor). Otherwise, calls that pass lambdas would become ambiguous.
• We discourage use of our transform and filter methods in favor of Stream nowadays. (We should really add a warning to Iterators similar to the one on https://guava.dev/Iterables.) It would be somewhat sad to make some of the discouraged methods in Iterators take up twice as much space in the Javadoc while leaving the handful that we still encourage alone.
• Migrating is usually still relatively straightforward because you can use function::apply to adapt back and forth. Still, it's clearly true that it would be better not to have to :)

[reschke]

Thanks for the pointers. That makes it indeed easier.