rlp module refactor

[debris]

should fix issues like #8031 or #8032

• rlp::decode should return Result<_, DecoderError>
• get rid of Rlp
• UntrustedRlp should be renamed to Rlp

[debris]

issues is marked as easy and epic, cause it's easy, but time consuming

[ascjones]

I'm looking at this

[ascjones]

This is being split across several PRs, since lots of changes

Phases:

• Replace 'trusted' usages of Rlp with UntrustedRlp and handle Results, unsafely unwrapping where appropriate
• Convert ethcore views to use UntrustedRlp and handle Results
• Delete legacy Rlp and rename UntrustedRlp to Rlp
• Change decode to return Result<_,DecoderError>

[ascjones]

Just the last bit needs doing now, changing decode to return Result<_, DecoderError>. I'll leave that here for now, I've unassigned myself for now so anyone else can pick it up or I'll come back to it later.

[dvdplm]

I'll have a stab at the decode changes. :)

[ascjones]

All done!