uniqueschars: use map cap based on input

unique-chars/main.go

@@ -1,9 +1,12 @@
 package uniqueschars
 
-import "unicode"
+import (
+	"unicode"
+	"unicode/utf8"
+)
 
 func Unique(s string) bool {
-	m := make(map[rune]struct{}, 50)
+	m := make(map[rune]struct{}, utf8.RuneCountInString(s)/2)
 
 	for _, c := range s {
 		c := unicode.ToLower(c)