consensus, core/*, params: metropolis preparation refactor

This commit is a preparation for the upcoming metropolis hardfork. It
prepares the state, core and vm packages such that integration with
metropolis becomes less of a hassle.

* Difficulty calculation requires header instead of individual
  parameters
* statedb.StartRecord renamed to statedb.Prepare and added Finalise
  method required by metropolis, which removes unwanted accounts from
  the state (i.e. selfdestruct)
* State keeps record of destructed objects (in addition to dirty
  objects)
* core/vm pre-compiles may now return errors
* core/vm pre-compiles gas check now take the full byte slice as argument
  instead of just the size
* core/vm now keeps several hard-fork instruction tables instead of a
  single instruction table and removes the need for hard-fork checks in
  the instructions
* core/vm contains a empty restruction function which is added in
  preparation of metropolis write-only mode operations
* Adds the bn256 curve
* Adds and sets the metropolis chain config block parameters (2^64-1)

miner/worker.go

@@ -523,7 +523,7 @@ func (env *Work) commitTransactions(mux *event.TypeMux, txs *types.TransactionsB
 			continue
 		}
 		// Start executing the transaction
-		env.state.StartRecord(tx.Hash(), common.Hash{}, env.tcount)
+		env.state.Prepare(tx.Hash(), common.Hash{}, env.tcount)
 
 		err, logs := env.commitTransaction(tx, bc, coinbase, gp)
 		switch err {