Decode BTreeMap/HashMap etc as Vec<(K,V)>

[jsdw]

A recent change in substrate meant that we ran into #454, which was worked around in #455 by adding a hardcoded type with custom derives.

As a more permanent fix, and a simpler solution than working out what custom traits to add to certain types, @ascjones suggested instead decoding types like HashMap<K,V> and BTreeMap<K,V> into Vec<(K,V)>.

The advantage of doing this is that we don't need any particular traits to be implemented for K and V, and additionally any custom ordering and such is preserved, so that we don't need to think about custom impls on the Substrate side for such things.

Once this is implemented, remove the hack in #455.