Refactor for speed optimizations...

Make the Clock type an int, rather than a struct
  Inline the hour/minute conversions on String()
  Inline the calculation of total and its modulo to minutesPerDay
  Benchmarks show ~35% improvement

bench1

@@ -0,0 +1,4 @@
+PASS
+BenchmarkAddMinutes-12  	50000000	        29.7 ns/op
+BenchmarkCreateClocks-12	50000000	        33.9 ns/op
+ok  	exercism/go/clock	3.252s

clock.go

@@ -1,57 +1,45 @@
+// Package clock represents a dateless time of day.
 package clock
 
 import "fmt"
 
 const (
 	testVersion    = 3
-	hoursPerDay    = 24
 	minutesPerHour = 60
 	minutesPerDay  = 1440
 )
 
 // A Clock stores a minute value representing a time of day.
-type Clock struct {
-	minutes int
-}
+type Clock int
 
 // String returns an hour:minute representation of the Clock.
 func (c Clock) String() string {
-	return fmt.Sprintf("%02d:%02d", c.hour(), c.mins())
+	return fmt.Sprintf("%02d:%02d", c/minutesPerHour, c%minutesPerHour)
 }
 
 // Add returns a Clock adjusted by the number of minutes specified.
 func (c Clock) Add(minutes int) Clock {
-	c.minutes = parseTime(0, c.minutes+minutes)
-	return c
-}
-
-// hour computes the hourly portion of the Clock.
-func (c Clock) hour() int {
-	return c.minutes / minutesPerHour
-}
-
-// mins computes the non-hourly remainder of the Clock.
-func (c Clock) mins() int {
-	return c.minutes % minutesPerHour
+	return parseTime(0, int(c)+minutes)
 }
 
 // New returns a Clock constructed from the given hour and minute values.
 func New(hour, minutes int) Clock {
-	return Clock{parseTime(hour, minutes)}
+	return parseTime(hour, minutes)
 }
 
 // parseTime computes the number of minutes that represent the time.
-func parseTime(hour, minutes int) int {
-	// normalize hours that are out of range [0-23]
-	hour = hour % hoursPerDay
-	// normalize any out of range values [0-1439]
-	minutes = minutes % minutesPerDay
-	// compute the total minutes
-	total := hour * minutesPerHour
-	total += minutes
-	// normalize any negative minute values
+func parseTime(hour, minutes int) Clock {
+	// Compute the total minutes, factoring away multiple day values.
+	total := (hour*minutesPerHour + minutes) % minutesPerDay
+	// If total minutes is negative, roll forward one day.
 	if total < 0 {
 		total += minutesPerDay
 	}
-	return total
+	return Clock(total)
 }
+
+// Benchmark versus previous iteration:
+//
+// benchmark                    old ns/op     new ns/op     delta
+// BenchmarkAddMinutes-12       45.7          29.7          -35.01%
+// BenchmarkCreateClocks-12     52.1          33.9          -34.93%