Feat/fractional units

[bitbeckers]

Branch of #72 so close that one first Implements and closes #73

• All tokens are minted as NFTs
• All tokens have 'units'
• Mint assigns units to tokens in addition to regular EIP1155 mint with supply of 1
• Merge transfers all units and burns the empty tokens
• Split mints new tokens with same base type ID and transfers units
• Cleaned up safeTransferFrom
• Bunch of optimisation TODOs
• Reference URI of base type. E.g. Call uri for fractional token 11, get uri for base type 10

[ryscheng]

For reference, cross-linking the discussion about what token interface to use https://github.com/Network-Goods/hypercerts/issues/51