use map[string]struct{} as a set.

Use map[string]struct{} instead of map[string]bool to implement a set
and reduce memory space.

inline.go

@@ -488,7 +488,7 @@ func link(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 			}
 
 			p.notes = append(p.notes, ref)
-			p.notesRecord[string(ref.link)] = true
+			p.notesRecord[string(ref.link)] = struct{}{}
 
 			link = ref.link
 			title = ref.title
@@ -502,7 +502,7 @@ func link(p *parser, out *bytes.Buffer, data []byte, offset int) int {
 			if t == linkDeferredFootnote && !p.isFootnote(lr) {
 				lr.noteId = len(p.notes) + 1
 				p.notes = append(p.notes, lr)
-				p.notesRecord[string(lr.link)] = true
+				p.notesRecord[string(lr.link)] = struct{}{}
 			}
 
 			// keep link and title from reference

markdown.go

@@ -219,7 +219,7 @@ type parser struct {
 	// presence. If a ref is also a footnote, it's stored both in refs and here
 	// in notes. Slice is nil if footnotes not enabled.
 	notes       []*reference
-	notesRecord map[string]bool
+	notesRecord map[string]struct{}
 }
 
 func (p *parser) getRef(refid string) (ref *reference, found bool) {
@@ -382,7 +382,7 @@ func MarkdownOptions(input []byte, renderer Renderer, opts Options) []byte {
 
 	if extensions&EXTENSION_FOOTNOTES != 0 {
 		p.notes = make([]*reference, 0)
-		p.notesRecord = make(map[string]bool)
+		p.notesRecord = make(map[string]struct{})
 	}
 
 	first := firstPass(p, input)