make size limit public

Signed-off-by: Jay Lee <BusyJayLee@gmail.com>

src/codec.rs

@@ -8,7 +8,7 @@ pub type SerializeFn<T> = fn(&T, &mut Vec<u8>) -> Result<()>;
 
 /// According to https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md, grpc uses
 /// a four bytes to describe the length of a message, so it should not exceed u32::MAX.
-const MAX_MESSAGE_SIZE: usize = u32::MAX as usize;
+pub const MAX_MESSAGE_SIZE: usize = u32::MAX as usize;
 
 /// Defines how to serialize and deserialize between the specialized type and byte slice.
 pub struct Marshaller<T> {