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/virtual_machine.go

@@ -0,0 +1,7 @@
+package ethvm
+
+type VirtualMachine interface {
+	Env() Environment
+	RunClosure(*Closure) ([]byte, error)
+	Depth() int
+}