Better return values from range/prefix

[ethanfrey]

Builds on #197

When iterating over a range from a Prefix we get from Map, we get back (Vec<u8> , T). However, we know what the Suffix is.Eg for K = (&[u8], U32Key) we could get back (U32Key, T)

[ethanfrey]

When we do implement this, we should:

1. Add a new trait (ParsableKey ?) with this method, it would not be required for all keys
2. Add a new method to Map, etc like range_parsed that returns (K, T) not (Vec<u8>, T) but only when K implements ParseableKey.
3. Implement something like Take storage keys as reference (K -> &K) #265 so we can return an Owned structure from parsing, but pass in a ref to query.

[ethanfrey]

Question: what happens when the primary key is references, eg. &Addr, or &str? It is relatively clear how to do so when the key is owned like eg. String or U32Key. Let's figure out how to define this better