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)

internal/ethapi/tracer_test.go

@@ -48,7 +48,7 @@ func runTrace(tracer *JavascriptTracer) (interface{}, error) {
 	contract := vm.NewContract(account{}, account{}, big.NewInt(0), 10000)
 	contract.Code = []byte{byte(vm.PUSH1), 0x1, byte(vm.PUSH1), 0x1, 0x0}
 
-	_, err := env.Interpreter().Run(contract, []byte{})
+	_, err := env.Interpreter().Run(0, contract, []byte{})
 	if err != nil {
 		return nil, err
 	}