common/types: add Address.Big (ethereum#26132)

Many of the other types have a function to convert the type to a big.Int,
but Address was missing this function.

It is useful to be able to turn an Address into a big.Int when doing
EVM-like computations natively in Go. Sometimes a Solidity address
type is casted to a uint256 and having a Big method on the Address
type makes this easy.

common/types.go

@@ -231,6 +231,9 @@ func (a Address) Bytes() []byte { return a[:] }
 // Hash converts an address to a hash by left-padding it with zeros.
 func (a Address) Hash() Hash { return BytesToHash(a[:]) }
 
+// Big converts an address to a big integer.
+func (a Address) Big() *big.Int { return new(big.Int).SetBytes(a[:]) }
+
 // Hex returns an EIP55-compliant hex string representation of the address.
 func (a Address) Hex() string {
 	return string(a.checksumHex())