cmd/evm, core/vm, test: refactored VM and core

* Moved `vm.Transfer` to `core` package and changed execution to call
`env.Transfer` instead of `core.Transfer` directly.
* core/vm: byte code VM moved to jump table instead of switch
* Moved `vm.Transfer` to `core` package and changed execution to call
  `env.Transfer` instead of `core.Transfer` directly.
* Byte code VM now shares the same code as the JITVM
* Renamed Context to Contract
* Changed initialiser of state transition & unexported methods
* Removed the Execution object and refactor `Call`, `CallCode` &
  `Create` in to their own functions instead of being methods.
* Removed the hard dep on the state for the VM. The VM now
  depends on a Database interface returned by the environment. In the
  process the core now depends less on the statedb by usage of the env
* Moved `Log` from package `core/state` to package `core/vm`.

core/vm/gas.go

@@ -37,6 +37,7 @@ var (
 	GasContractByte = big.NewInt(200)
 )
 
+// baseCheck checks for any stack error underflows
 func baseCheck(op OpCode, stack *stack, gas *big.Int) error {
 	// PUSH and DUP are a bit special. They all cost the same but we do want to have checking on stack push limit
 	// PUSH is also allowed to calculate the same price for all PUSHes
@@ -63,6 +64,7 @@ func baseCheck(op OpCode, stack *stack, gas *big.Int) error {
 	return nil
 }
 
+// casts a arbitrary number to the amount of words (sets of 32 bytes)
 func toWordSize(size *big.Int) *big.Int {
 	tmp := new(big.Int)
 	tmp.Add(size, u256(31))