Improve extensibility of ERC1155

@KaiRo-at suggested that our current contract is not extensible enough due to using external functions.

They also suggested to add an internal function that implements transfers to have a central extensibility point. I think this is fine but I would like to see the internal function use msg.sender rather than having a sender argument like we did in ERC20.

[Cybourgeoisie]

Hey all - I’d like to propose setting virtual to uri and isApprovedForAll in the ERC1155.sol file. OpenSea currently relies on an override to the default isApprovedForAll to permit use of a proxy address, and OpenSea relies on uri returning the id value concatenated to the URI.

[KaiRo-at]

Hmm, if enough virtuals are spread throughout this and people can override those functions (I forgot for a moment that functions marked virtual do not actually require an override), then it may still work and be helpful, yes.