core/vm: Refactor tracing to make Tracer the main interface

This CL makes several refactors:
 - Define a Tracer interface, implementing the `CaptureState` method
 - Add the VM environment as the first argument of
   `Tracer.CaptureState`
 - Rename existing functionality `StructLogger` an make it an
   implementation of `Tracer`
 - Delete `StructLogCollector` and make `StructLogger` collect the logs
   directly
 - Change all callers to use the new `StructLogger` where necessary and
   extract logs from that.
 - Deletes the apparently obsolete and likely nonfunctional 'TraceCall'
   from the eth API.

Callers that only wish accumulated logs can use the `StructLogger`
implementation straightforwardly. Callers that wish to efficiently
capture VM traces and operate on them without excessive copying can now
implement the `Tracer` interface to receive VM state at each step and
do with it as they wish.

This CL also removes the accumulation of logs from the vm.Environment;
this was necessary as part of the refactor, but also simplifies it by
removing a responsibility that doesn't directly belong to the
Environment.

core/vm/stack.go

@@ -24,58 +24,58 @@ import (
 // stack is an object for basic stack operations. Items popped to the stack are
 // expected to be changed and modified. stack does not take care of adding newly
 // initialised objects.
-type stack struct {
+type Stack struct {
 	data []*big.Int
 }
 
-func newstack() *stack {
-	return &stack{}
+func newstack() *Stack {
+	return &Stack{}
 }
 
-func (st *stack) Data() []*big.Int {
+func (st *Stack) Data() []*big.Int {
 	return st.data
 }
 
-func (st *stack) push(d *big.Int) {
+func (st *Stack) push(d *big.Int) {
 	// NOTE push limit (1024) is checked in baseCheck
 	//stackItem := new(big.Int).Set(d)
 	//st.data = append(st.data, stackItem)
 	st.data = append(st.data, d)
 }
-func (st *stack) pushN(ds ...*big.Int) {
+func (st *Stack) pushN(ds ...*big.Int) {
 	st.data = append(st.data, ds...)
 }
 
-func (st *stack) pop() (ret *big.Int) {
+func (st *Stack) pop() (ret *big.Int) {
 	ret = st.data[len(st.data)-1]
 	st.data = st.data[:len(st.data)-1]
 	return
 }
 
-func (st *stack) len() int {
+func (st *Stack) len() int {
 	return len(st.data)
 }
 
-func (st *stack) swap(n int) {
+func (st *Stack) swap(n int) {
 	st.data[st.len()-n], st.data[st.len()-1] = st.data[st.len()-1], st.data[st.len()-n]
 }
 
-func (st *stack) dup(n int) {
+func (st *Stack) dup(n int) {
 	st.push(new(big.Int).Set(st.data[st.len()-n]))
 }
 
-func (st *stack) peek() *big.Int {
+func (st *Stack) peek() *big.Int {
 	return st.data[st.len()-1]
 }
 
-func (st *stack) require(n int) error {
+func (st *Stack) require(n int) error {
 	if st.len() < n {
 		return fmt.Errorf("stack underflow (%d <=> %d)", len(st.data), n)
 	}
 	return nil
 }
 
-func (st *stack) Print() {
+func (st *Stack) Print() {
 	fmt.Println("### stack ###")
 	if len(st.data) > 0 {
 		for i, val := range st.data {