Tune up static_buffer (API Change):

The static buffer is updated:

* reset() is no longer a member. Use b.consume(b.size()) instead.

* Simplified implementaton, uses asio instead of custom types

* Better stream performance: consuming the input makes room
  available in the output. This class is now suitable for
  HTTP reads.

These changes permit the static_buffer wrapper to adapt a user
memory buffer if desired, including a stack based array.
The static_buffer_n class may also be used for this purpose,
it comes with its own storage.

include/beast/core/flat_buffer.hpp

@@ -75,7 +75,7 @@ private:
         return static_cast<std::size_t>(last - first);
     }
 
-    char* p_;
+    char* begin_;
     char* in_;
     char* out_;
     char* last_;
@@ -218,7 +218,7 @@ public:
     std::size_t
     capacity() const
     {
-        return dist(p_, end_);
+        return dist(begin_, end_);
     }
 
     /// Get a list of buffers that represent the input sequence.