add Identity.IsZero method

This commit adds the `IsZero() bool` method to the `Identity` type.
It returns true if the identity is equal to the Identity zero value.
A 'valid' identity won't be zero since `H(data)` will not produce
a hash value of all zero bits with overwhelming probability if a H
is a collision-resistant hash function.

The `IsZero` method is inline with the accepted Go JSON proposal
adding the struct tag `omitzero`.
Ref: golang/go#45669 (comment)

Signed-off-by: Andreas Auernhammer <[email protected]>

identity.go

@@ -49,6 +49,11 @@ type Identity struct {
 	hash [32]byte
 }
 
+var zeroIdentity = Identity{}
+
+// IsZero returns true if i is the Identity zero value.
+func (i Identity) IsZero() bool { return i == zeroIdentity }
+
 // MarshalBinary returns a binary representation of the identity.
 func (i Identity) MarshalBinary() ([]byte, error) {
 	var buf [35]byte