Mutexes: Advise against embedding (#130)

Our guidance for mutexes suggested that it was okay to embed mutexes in
structs if the struct was unexported. This was left as an allowance for
"if you really want to do it, this is the only time it's okay", but that
nuance doesn't translate well to the style guide.

Advise against embedding mutexes completely to resolve any confusion
here.

Resolves #127

style.md

@@ -332,29 +332,28 @@ mu.Lock()
 </td></tr>
 </tbody></table>
 
-If you use a struct by pointer, then the mutex can be a non-pointer field.
-
-Unexported structs that use a mutex to protect fields of the struct may embed
-the mutex.
+If you use a struct by pointer, then the mutex should be a non-pointer field on
+it. Do not embed the mutex on the struct, even if the struct is not exported.
 
 <table>
+<thead><tr><th>Bad</th><th>Good</th></tr></thead>
 <tbody>
 <tr><td>
 
 ```go
-type smap struct {
-  sync.Mutex // only for unexported types
+type SMap struct {
+  sync.Mutex
 
   data map[string]string
 }
 
-func newSMap() *smap {
-  return &smap{
+func NewSMap() *SMap {
+  return &SMap{
     data: make(map[string]string),
   }
 }
 
-func (m *smap) Get(k string) string {
+func (m *SMap) Get(k string) string {
   m.Lock()
   defer m.Unlock()
 
@@ -387,12 +386,17 @@ func (m *SMap) Get(k string) string {
 
 </td></tr>
 
-</tr>
-<tr>
-<td>Embed for private types or types that need to implement the Mutex interface.</td>
-<td>For exported types, use a private field.</td>
-</tr>
+<tr><td>
 
+The `Mutex` field, and the `Lock` and `Unlock` methods are unintentionally part
+of the exported API of `SMap`.
+
+</td><td>
+
+The mutex and its methods are implementation details of `SMap` hidden from its
+callers.
+
+</td></tr>
 </tbody></table>
 
 ### Copy Slices and Maps at Boundaries