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.go

@@ -0,0 +1,32 @@
+// Package set implement sets for various types. Well, only string for now :)
+package set
+
+type String struct {
+	m map[string]struct{}
+}
+
+func NewString(values ...string) *String {
+	s := &String{}
+	s.Add(values...)
+	return s
+}
+
+func (s *String) Add(values ...string) {
+	if s.m == nil {
+		s.m = map[string]struct{}{}
+	}
+
+	for _, v := range values {
+		s.m[v] = struct{}{}
+	}
+}
+
+func (s *String) Has(value string) bool {
+	// We explicitly allow s to be nil *in this function* to simplify callers'
+	// code.  Note that Add will not tolerate it, and will panic.
+	if s == nil || s.m == nil {
+		return false
+	}
+	_, ok := s.m[value]
+	return ok
+}