//ERC-20 tokens must have 6 functions.
TotalSupply: provides information about the total token supply
BalanceOf: provides account balance of the owner's account
Transfer: executes transfers of a specified number of tokens to a specified address
TransferFrom: executes transfers of a specified number of tokens from a specified address
Approve: allow a spender to withdraw a set number of tokens from a specified account
Allowance: returns a set number of tokens from a spender to the owner