Avoid a parser allocation using non-flat buffer:

When the parser is presented with an input buffer
sequence with length greater than one, the input is
flattened using stack space instead of dynamic
allocation if the buffer size is no greater than 8192.

include/beast/http/basic_parser.hpp

@@ -74,6 +74,9 @@ class basic_parser
     // limit on the size of the obs-fold buffer
     static std::size_t constexpr max_obs_fold = 4096;
 
+    // limit on the size of the stack flat buffer
+    static std::size_t constexpr max_stack_buffer = 8192;
+
     // Message will be complete after reading header
     static unsigned constexpr flagSkipBody              = 1<<  0;
 
@@ -363,9 +366,10 @@ private:
     }
 
     template<class ConstBufferSequence>
-    string_view
-    maybe_flatten(
-        ConstBufferSequence const& buffers);
+    std::size_t
+    put_from_stack(std::size_t size,
+        ConstBufferSequence const& buffers,
+            error_code& ec);
 
     void
     parse_header(char const*& p,