accounts/abi/bind/v2: add Address method to BoundContract

[tr1sm0s1n]

Rationale

Since we're already storing the deployment address inside BoundContract, a simple reader method would be helpful to access the same without allocating new variables.

For example, here's the PackConstructor method for the OpenZeppelin Governance contract:

func (myGovernor *MyGovernor) PackConstructor(_token common.Address, _timelock common.Address) []byte {
	enc, err := myGovernor.abi.Pack("", _token, _timelock)
	if err != nil {
		panic(err)
	}
	return enc
}

Since we've already deployed the token and timelock contracts and created instance wrappers for the same, simply calling Address() on those wrappers will be helpful here.