[#3] signature: Refactor sign and verify

Split methods to separate files, drop redundant intefaces

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

signature/marshaller.go

@@ -0,0 +1,26 @@
+package signature
+
+type stableMarshaler interface {
+	StableMarshal([]byte) []byte
+	StableSize() int
+}
+
+type StableMarshalerWrapper struct {
+	SM stableMarshaler
+}
+
+func (s StableMarshalerWrapper) ReadSignedData(buf []byte) ([]byte, error) {
+	if s.SM != nil {
+		return s.SM.StableMarshal(buf), nil
+	}
+
+	return nil, nil
+}
+
+func (s StableMarshalerWrapper) SignedDataSize() int {
+	if s.SM != nil {
+		return s.SM.StableSize()
+	}
+
+	return 0
+}