BaseNFT interfaces and helper contracts

[austinkline]

This change lays out the beginning steps to define a base layer of contract interfaces to make creating contracts easier. There are three contracts so far:

1. BaseNFT - Defines interfaces that extend NonFungibleToken.Collection and NonFungibleToken.NFT. These interfaces remove as much boilerplate code as they can from the concrete implementations of Collection and NFT that a creator would need to define.
2. NFTMetadata - A contract that defines types to more easily contain and vend metadata.
3. BaseNFTVars - A small contract interface to define pre-set variables that must be on a contract definition. These variables are used by BaseNFT to allow default implementations of metadata views

What we still need:

1. There needs to be a better way to initialize contracts, the init function is ~50% of our OpenEditionNFT definition now. If we can make those inputs, we can remove the majority of the setup needed to get working.
2. A universal collection implementation. This collection should only allow one type of NFT in it so that things do not get confused/overridden with other types, but can still be used to remove code from a creator's code
3. A way to remove Minter boilerplate code. Currently the Minter interface defined in our drops is unchanged in our OpenEditionNFT definition. But we will need to abstract some logic away into interfaces so that we can define ways to mint (sequential, random, etc.) while also removing more duplicate code from contracts we generate
4. A contract factory that can be given a contract name and the type of contract it should be (probably based on what it should inherit), and makes a new NFT collection fully configured and ready to begin the minting process
5. A contract manager which helps a creator access contracts while not requiring them to exist on the creator's account itself. It should delegate access, instead.