Fully generalize `BTree{Map, Set}` range iterators

This permits collections with `String` keys to be ranged over with
`&str` bounds. The `K` defaults for `Min` and `Max` permit the default
type parameter fallback to work with things like

```rust
use std::collections::{BTreeSet, Bound};
let set = BTreeSet::<String>::new();
set.range(Bound::Included("a"), Bound::Unbounded);
```

Without the defaults, the type of the maximum bound would be
unconstrained.

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