Implement a string set data type

We will need a few string sets in different places, create it as a separate
package for convenience.

For now, it only implements strings, but it may be extended to other data
types in the future.

internal/set/set_test.go

@@ -0,0 +1,40 @@
+package set
+
+import "testing"
+
+func TestString(t *testing.T) {
+	s1 := &String{}
+
+	// Test that Has works on a new set.
+	if s1.Has("x") {
+		t.Error("'x' is in the empty set")
+	}
+
+	s1.Add("a")
+	s1.Add("b", "ccc")
+
+	expectStrings(s1, []string{"a", "b", "ccc"}, []string{"notin"}, t)
+
+	s2 := NewString("a", "b", "c")
+	expectStrings(s2, []string{"a", "b", "c"}, []string{"notin"}, t)
+
+	// Test that Has works (and not panics) on a nil set.
+	var s3 *String
+	if s3.Has("x") {
+		t.Error("'x' is in the nil set")
+	}
+}
+
+func expectStrings(s *String, in []string, notIn []string, t *testing.T) {
+	for _, str := range in {
+		if !s.Has(str) {
+			t.Errorf("String %q not in set, it should be", str)
+		}
+	}
+
+	for _, str := range notIn {
+		if s.Has(str) {
+			t.Errorf("String %q is in the set, should not be", str)
+		}
+	}
+}