Use sync.Map instead of mutex synchronization for mapper cache

There was a lot of contention on the cache mutex. At the same
time the cache only grows. This is the case that sync.Map was created
for, so use it instead to improve performance.

After applying this patch, we no longer see the contention
in the profiler.

Fixes #2

reflect.go

@@ -66,18 +66,16 @@ func (f StructMap) GetByTraversal(index []int) *FieldInfo {
 // behaves like most marshallers in the standard library, obeying a field tag
 // for name mapping but also providing a basic transform function.
 type Mapper struct {
-	cache      map[reflect.Type]*StructMap
+	cache      sync.Map // map[reflect.Type]*StructMap
 	tagName    string
 	tagMapFunc func(string) string
 	mapFunc    func(string) string
-	mutex      sync.Mutex
 }
 
 // NewMapper returns a new mapper using the tagName as its struct field tag.
 // If tagName is the empty string, it is ignored.
 func NewMapper(tagName string) *Mapper {
 	return &Mapper{
-		cache:   make(map[reflect.Type]*StructMap),
 		tagName: tagName,
 	}
 }
@@ -87,7 +85,6 @@ func NewMapper(tagName string) *Mapper {
 // have values like "name,omitempty".
 func NewMapperTagFunc(tagName string, mapFunc, tagMapFunc func(string) string) *Mapper {
 	return &Mapper{
-		cache:      make(map[reflect.Type]*StructMap),
 		tagName:    tagName,
 		mapFunc:    mapFunc,
 		tagMapFunc: tagMapFunc,
@@ -99,7 +96,6 @@ func NewMapperTagFunc(tagName string, mapFunc, tagMapFunc func(string) string) *
 // for any other field, the mapped name will be f(field.Name)
 func NewMapperFunc(tagName string, f func(string) string) *Mapper {
 	return &Mapper{
-		cache:   make(map[reflect.Type]*StructMap),
 		tagName: tagName,
 		mapFunc: f,
 	}
@@ -108,14 +104,13 @@ func NewMapperFunc(tagName string, f func(string) string) *Mapper {
 // TypeMap returns a mapping of field strings to int slices representing
 // the traversal down the struct to reach the field.
 func (m *Mapper) TypeMap(t reflect.Type) *StructMap {
-	m.mutex.Lock()
-	mapping, ok := m.cache[t]
-	if !ok {
-		mapping = getMapping(t, m.tagName, m.mapFunc, m.tagMapFunc)
-		m.cache[t] = mapping
+	if mapping, ok := m.cache.Load(t); ok {
+		return mapping.(*StructMap)
 	}
-	m.mutex.Unlock()
-	return mapping
+
+	mapping, _ := m.cache.LoadOrStore(t, getMapping(t, m.tagName, m.mapFunc, m.tagMapFunc))
+
+	return mapping.(*StructMap)
 }
 
 // FieldMap returns the mapper's mapping of field names to reflect values.  Panics