Add a count method to Source

[KodrAus]

Adds a Source::count(&self) -> usize method to figure out how many key-value pairs to expect.

I've also tried to clarify the expectation that subsequent calls to Source::visit will yield the same key-value pairs unless the visitor fails. A buggy Source::visit with the default implementation of count may yield a different number of key-value pairs. Given that, we could make the method something like Source::size_hint(&self) -> Option<usize> instead, but I think the fundamental requirement that Source::visit passes the same pairs to the visitor unless the visitor itself fails is reasonable and lets us add more useful methods like for_each that don't have to return results. The reason visit itself is fallible is so that a formatting or serialization error can be passed upstream for a logging framework to surface in some way.

[KodrAus]

r? @Thomasdezeeuw

[Thomasdezeeuw]

Looks good to me.