Use associated type for Iterator trait

[Fullstop000]

Current Iterator trait just returns Slice in key() and value()

wickdb/src/iterator.rs

Lines 59 to 65 in 6029534

	fn key(&self) -> Slice;
	/// Return the value for the current entry. The underlying storage for
	/// the returned slice is valid only until the next modification of
	/// the iterator.
	/// REQUIRES: `valid()`
	fn value(&self) -> Slice;

This can limit the usage of Iterators. We might want a non-slice key or value.

Consider using an advanced trait with associated type instead:

pub trait Iterator {
    type Key;
    type Value;

    fn key(&self) -> Self::Key;
    fn value(&self) -> Self::Value;
}

Block #49