ERC-1155 example

[kaihirota]

The sample game in this branch uses three collections / contracts:

1. ERC-721 Immutable Runner Fox collection: Fox NFT minted to players when they log in. Assuming this has no intrinsic value, this collection can be moved to ERC-1155.
2. ERC-20 Immutable Runner Token contract: Coins collected in-game are minted to the player. When a player has 3 or more coins, they can invoke craftSkin() contract function, which burns 3 of the caller's coins, and then mint a new skin in return from the collection below.
3. ERC-721 Immutable Runner Fox Skin: The above ERC-20 contract is the minterRole of this contract. When craftSkin() is called on the ERC-20 contract above, that contract will call the mint function on this contract.

In this branch, 1 and 2 above have been replaced by a single ERC-1155 contract. TokenID=1 in this collection is equivalent to "ERC-721 Immutable Runner Fox collection", and tokenID=2 is equivalent to "ERC-20 Immutable Runner Token". The RunnerToken1155 Solidity contract has two constants (FOXTOKENID, COINTOKENID) which are used in the mintNFT and mintCoins functions.