Optimize NFT contract code

[whoabuddy]

Looking over the default template, there are a few things:

• remove MAX-SUPPLY and references
• WALLET can be removed in favor of BTC receiver address (Add swap parameters to NFT contract #21)
• ERR-SOLD-OUT may not be necessary
• claim should be renamed to eliminate confusion on what it does for the person running it (maybe create or process)
• mint-many should be converted into a single mint function instead of iterating over a list

Thinking some more on this - maybe claim and mint should be combined into one public function? It would accept the BTC TX, and as long as it's valid per the requirements, complete the mint. Will definitely tie into #18.