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)

consensus/ethash/consensus_test.go

@@ -23,6 +23,7 @@ import (
 	"testing"
 
 	"github.com/ethereum/go-ethereum/common/math"
+	"github.com/ethereum/go-ethereum/core/types"
 	"github.com/ethereum/go-ethereum/params"
 )
 
@@ -71,7 +72,11 @@ func TestCalcDifficulty(t *testing.T) {
 	config := &params.ChainConfig{HomesteadBlock: big.NewInt(1150000)}
 	for name, test := range tests {
 		number := new(big.Int).Sub(test.CurrentBlocknumber, big.NewInt(1))
-		diff := CalcDifficulty(config, test.CurrentTimestamp, test.ParentTimestamp, number, test.ParentDifficulty)
+		diff := CalcDifficulty(config, test.CurrentTimestamp, &types.Header{
+			Number:     number,
+			Time:       test.ParentTimestamp,
+			Difficulty: test.ParentDifficulty,
+		})
 		if diff.Cmp(test.CurrentDifficulty) != 0 {
 			t.Error(name, "failed. Expected", test.CurrentDifficulty, "and calculated", diff)
 		}