Allow creation of contract instances from address

[AuHau]

If token uses "instance" of other contracts which are "linked" using address, then it would be nice to have ability to decode the address into contract's instance instead of getting the address. For example:

type TestContract = ref object of Contract
proc token(_: TestContract): Erc20Token {.contract, view.}

For that to allow, we need to have ability to get a Provider or Signer in the decode() function though.