Rework HTTP concepts (API Change):

fix #139, fix #140

* Writer uses write instead of operator()
* Refactor traits to use void_t
* Remove is_ReadableBody, is_WritableBody
* Add has_reader, has_writer, is_Reader, is_Writer
* More friendly compile errors on failed concept checks

examples/file_body.hpp

@@ -58,14 +58,15 @@ struct file_body
         }
 
         std::uint64_t
-        content_length() const
+        content_length() const noexcept
         {
             return size_;
         }
 
-        template<class Write>
+        template<class WriteFunction>
         boost::tribool
-        operator()(resume_context&&, error_code&, Write&& write)
+        write(resume_context&&, error_code&,
+            WriteFunction&& wf) noexcept
         {
             if(size_ - offset_ < sizeof(buf_))
                 buf_len_ = static_cast<std::size_t>(
@@ -75,7 +76,7 @@ struct file_body
             auto const nread = fread(buf_, 1, sizeof(buf_), file_);
             (void)nread;
             offset_ += buf_len_;
-            write(boost::asio::buffer(buf_, buf_len_));
+            wf(boost::asio::buffer(buf_, buf_len_));
             return offset_ >= size_;
         }
     };