RLP: adapting to changes in Solidity version 0.5.0

[christianparpart]

This PR adapts the RLP.sol to the changes needed when you compile this file with a solidity compiler version 0.5.0+, which is not yet released, but you can test it already with current compiler that has experimental v0.5.0 features enabled.

This PR basically contains the changes we did in order to get our test suite pass.

[axic]

Shouldn't this also update the assembly?

[ekpyron]

I vote for updating the assembly and promoting _copyToBytes back to view (both here and in the solidity repo; actually my fault - I should have done that with the constant keyword PR already).