rlp: new package for streaming RLP decoder

This is the implementation of the decoder that I talked about last week.
We can add the encoder later, when the mapping is discussed.

For the supported types, please look at the comment of `Decode` and give feedback.

A couple of types that could be handled are not implemented right now
because I don't know what the mapping should be and whether there is
any need for them.
 
- `bool`: we could go with `v != 0`
- floating point numbers: do we need them at all?
- maps: the obvious encoding would be `[["foo", ...], ["bar", ...]]`.  
  we'd need to restrict the key types.