maputil: preallocate len and cap for keys and values slice (#41)

Length of keys and values is know in advance, use this knowledge to preallocate memory.
Space usage increases by const factor

maputil/map.go

@@ -8,10 +8,12 @@ import "reflect"
 
 // Keys returns a slice of the map's keys
 func Keys[K comparable, V any](m map[K]V) []K {
-	keys := make([]K, 0, len(m))
+	keys := make([]K, len(m))
 
+	var i int
 	for k := range m {
-		keys = append(keys, k)
+		keys[i] = k
+		i++
 	}
 
 	return keys
@@ -19,10 +21,12 @@ func Keys[K comparable, V any](m map[K]V) []K {
 
 // Values returns a slice of the map's values
 func Values[K comparable, V any](m map[K]V) []V {
-	values := make([]V, 0, len(m))
+	values := make([]V, len(m))
 
+	var i int
 	for _, v := range m {
-		values = append(values, v)
+		values[i] = v
+		i++
 	}
 
 	return values