[#3] signature: Refactor sign and verify

Split methods to separate files, drop redundant intefaces

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

util/collection/slice.go

@@ -0,0 +1,17 @@
+package collection
+
+import "golang.org/x/exp/constraints"
+
+// Max finds maximum value of items.
+func Max[T constraints.Ordered](items ...T) T {
+	if len(items) == 0 {
+		panic("failed to get max value: empty slice")
+	}
+	result := items[0]
+	for i := 1; i < len(items); i++ {
+		if items[i] > result {
+			result = items[i]
+		}
+	}
+	return result
+}