Remove Keymappable protocol

[chriscombs]

Keymappable and Decodable are always used together and I can't think of a single use case where you would want one but not the other. Keymappable can be removed and its functions added to Decodable instead, which makes sense because Decodable does nothing but add the array function and all of the actual decoding is done in Keymappable. I think it helps clear up the code a little bit, so that it's clear that there are only two main features, Encoding and Decoding.

[chriscombs]

Fixed in #151