RLP StreamReader

### What feature should we add?

While messages from a stream might normally need delineation, the length prefix of RLP makes delimiters redundant. Also, no delimiter can be valid for RLP. I am requesting a way to read complete rlp-encoded messages from a stream (eg `sys.stdin`).

Essentially it should work similar to [`asyncio.StreamReader.readline()`](https://docs.python.org/3/library/asyncio-stream.html#asyncio.StreamReader.readline) but read an entire RLP message rather than a utf-8 line.

## Usage
```
reader = rlp.StreamReader()
pipe_protocol = asyncio.StreamReaderProtocol(reader)
loop = asyncio.get_event_loop()
await loop.connect_read_pipe(
lambda: pipe_protocol, sys.stdin
)
message = await reader.read()
```