Refactored VM to two separate VMs; std & debug

Standard VM should be about 10x faster than the debug VM. Some error
checking has been removed, all of the log statements and therefor quite
some unnecessary if-statements.

ethvm/types.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 )
 
-type OpCode int
+type OpCode byte
 
 // Op codes
 const (