Grab bag of fixes

The test now checks that the number of of addresses is correct
and terminates cleanly.

It had been moved to Peer, probably for debugging.

...and make it a top-level function instead.

The original idea behind having EncodeMsg in the interface was that
implementations might be able to encode RLP data to their underlying
writer directly instead of buffering the encoded data. The encoder
will buffer anyway, so that doesn't matter anymore.

Given the recent problems with EncodeMsg (copy-pasted implementation
bug) I'd rather implement once, correctly.

Fixes the build. AFAIK evm does not bother keeping a chain and
cannot provide a real implementation.

fixes for rlpdump

cmd/rlpdump: new helper command

* Added pre-compiled contract 0x04 (mem cpy)
* Changed depth error to return the gas instead of consuming

* PREVHASH => BLOCKHASH( N )
* State transition object uses VMEnv as it's query interface
* Updated vm.Enviroment has GetHash( n ) for BLOCKHASH instruction
* Added GetHash to xeth, core, utils & test environments