Problem:
- The implementation of `find` is coupled to the implementation of `Vector`.
- `Vector::find` takes the predicate by value which might be expensive.
Solution:
- Decouple the implementation of `find` from `Vector` by using a
generic `find` algorithm.
- Change the name of `find` with a predicate to `find_if` so that a
binding reference can be used and the predicate can be forwarded to
avoid copies.
- Change all the `find(pred)` call sites to use `find_if`.
