ERC-1155 Example

[HCastano]

This is getting big, so I think it's a good time to get some feedback and also get
some of my questions answered.

This PR aims to inkplement ERC-1155 as specified here.

It implements the "core" APIs, the ERC1155 Receivable interface (even though the just
implementation panics and rejects tokens, lol), and also has some additional
functionality such as a way to easily mint tokens.

Note that this contract implementation is missing some extensions which are mentioned in
the ERC-1155 specification document:

• Missing ERC1155 Metadata URI Interface
• No differentiation between fungible and non-fungible tokens
• Tokens are not officially burnable
• No batch minting

I won't be adding these here since a) this PR is already pretty big and b) they could
make for good first contributor issues.

I also think there are some improvements to be made in terms of how "Rusty" some
things are (having events return None vs. 0x00), but I don't want to deviate too
far from the ERC-1155 spec.

[Robbepop]

The most important thing to deal with in this PR is how we do error handling. Currently the implementation panics a lot which generally is not ideal. Instead Result should be returned with a predefined set of potential errors (given an enum or so) so that depending contracts can handle those errors if they want to.

[HCastano]

So I think the disconnect with the Err vs. panic thing is that the spec explicitly says things like: "MUST revert if _to is the zero address.", which in my mind translates directly to panic and don't let the caller handle it.

However, after some thinking I agree, for most of these scenarios it's probably OK to let the caller decide how to proceed, and it also makes the code more idiomatic.

[Robbepop]

LGTM