Debug impl for HashMap::keys should not require values to be Debug

[jonhoo]

#39002 landed Debug implementations for a number of derived types in collections, but imposes overly strict requirements. For example, for std::collections::hash_map::Keys:

impl<'a, K: Debug, V: Debug> Debug for Keys<'a, K, V> { /* ... */ }

Given that only the keys are printed, we should only require K: Debug, not also V: Debug (unless I'm mistaken?).

[sfackler]

Yep, seems overly restrictive.

[jonhoo]

While we're on it, Values should only require V: Debug, and the same change should be applied to BTreeMap and friends.