Without these added transanction/header import/exports decoding
blocks or block bodies would fail if one only had imported
blocks_rlp.
Also removes unused import + add copyright comments in rlp code.
A first step in cleaning up RLP, which has lots of interesting issues -
the next step would be to clean up the exception handling as well
(Resultify?)
* remove `RlpNode` (unused)
* single-pass parsing for most functionality via RlpItem
* stricter conformance to spec
* remove float support
* warn about signed integers
* raise on invalid RLP earlier
* avoid several pointless allocations, in particular in `listLen`,
`listElem` etc
* include spec docs