Use memcpy to avoid putting one byte at a time when copying blocks

`memcpy` is likely to be faster than the naive method due to the
possibility of use of vector instructions and copying more than a byte
at a time.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

include/boost/beast/zlib/detail/deflate_stream.ipp

@@ -1298,9 +1298,8 @@ copy_block(
         put_short((std::uint16_t)len);
         put_short((std::uint16_t)~len);
     }
-    // VFALCO Use memcpy?
-    while (len--)
-        put_byte(*buf++);
+    std::memcpy(&pending_buf_[pending_], buf, len);
+    pending_ += len;
 }
 
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