rename the backing buffer property to "ref.buffer"

The old prop name "buffer" was very likely to collide, as it's a common
struct property name in C. The new name "ref.buffer" is not a valid C struct
name, so it's very unlikely to every be a problem in the future.

/cc @tjfontaine

lib/struct.js

@@ -10,7 +10,7 @@
  *
  * The only verboten field names are "ref", which is used used on struct
  * instances as a function to retrieve the backing Buffer instance of the
- * struct, and "buffer" which contains the backing Buffer instance.
+ * struct, and "ref.buffer" which contains the backing Buffer instance.
  *
  *
  * Example:
@@ -92,7 +92,7 @@ function Struct () {
 
     // set the backing Buffer store
     store.type = StructType
-    this.buffer = store
+    this['ref.buffer'] = store
 
     if (arg) {
       for (var key in arg) {
@@ -167,7 +167,7 @@ function set (buffer, offset, value) {
   }
   var isStruct = value instanceof this
   if (isStruct) {
-    value.buffer.copy(buffer, offset, 0, this.size);
+    value['ref.buffer'].copy(buffer, offset, 0, this.size);
   } else {
     new this(buffer, value)
   }
@@ -214,11 +214,11 @@ function defineProperty (name, type) {
   var desc = { enumerable: true , configurable: true }
   desc.get = function () {
     debug('getting "%s" struct field (offset: %d)', name, field.offset)
-    return ref.get(this.buffer, field.offset, type)
+    return ref.get(this['ref.buffer'], field.offset, type)
   }
   desc.set = function (value) {
     debug('setting "%s" struct field (offset: %d)', name, field.offset, value)
-    return ref.set(this.buffer, field.offset, value, type)
+    return ref.set(this['ref.buffer'], field.offset, value, type)
   }
 
   // calculate the new size and field offsets
@@ -301,7 +301,7 @@ var proto = {}
  * throw an error if you try to define a struct field with the name "buffer".
  */
 
-proto.buffer = ref.NULL
+proto['ref.buffer'] = ref.NULL
 
 /**
  * Flattens the Struct instance into a regular JavaScript Object. This function
@@ -345,5 +345,5 @@ proto.inspect = function inspect () {
  */
 
 proto.ref = function ref () {
-  return this.buffer
+  return this['ref.buffer']
 }