`model` uses `crate::ScriptPubkey` but it is not strongly typed

[tcharding]

model types are supposed to be strongly typed using types from rust-bitcoin however we are using the crate::ScritpPubkey type.

Found because @benthecarman wants to make the conversion getters public in #368

E.g. model::blockchain::SpendActivity

If I remember correctly when we added this I was lazy and did not know exactly how to convert into a model type (eg should we keep the asm field or use ScriptBuf::to_asm_string()).

Think through this properly and probably add a model type and implement into_model on crate::ScriptPubkey.

[jamillambert]

I have added a model for ScriptPubKey in #370.

One thing I noticed though is GetTxOut has a ScriptPubKey field but it doesn't keep it in the model. Instead it creates TxOut and Address type fields. Should this be updated to use the new model? and if so should we keep the TxOut and Address fields, which are convenient to use?

[tcharding]

I'm inclined to leave it like it is, one can grab the script_pubkey from the TxOut.

[jamillambert]

Closed by #370