pkg/scale: add support for maps

[qdm12]

As defined: https://spec.polkadot.network/#defn-scale-dictionary

We have a (bad but easily fixed) implementation in func (d *InherentsData) Encode() ([]byte, error) right now, and this one should be moved in the scale codec.

It has to be fixed to encode the key and value as tuples instead of writing them directly and pushing the encoding responsibility to the caller.

The function func (d *InherentsData) Encode() can then be removed and replaced with the scale package Marshal function

[qdm12]

Duplicate of #2796