feat: add hash struct

hash/errors.go

@@ -0,0 +1,22 @@
+package hash
+
+import (
+	"fmt"
+)
+
+// ErrNotStringer is returned when there's an error with hash:"string"
+type ErrNotStringer struct {
+	Field string
+}
+
+// Error implements error for ErrNotStringer
+func (ens *ErrNotStringer) Error() string {
+	return fmt.Sprintf("hash: %s has hash:\"string\" set, but does not implement fmt.Stringer", ens.Field)
+}
+
+// ErrFormat is returned when an invalid format is given to the Hash function.
+type ErrFormat struct{}
+
+func (*ErrFormat) Error() string {
+	return "format must be one of the defined Format values in the hash library"
+}