interp: fix the behaviour of goto, continue and break

[mvertes]

The logic of goto was false due to mixing break label and goto
label, despite being opposite. In case of break, jump to node (the
exit point) instead of node.start. Also always define label symbols
before their use is parsed.

Fixes #1354.

[mpl]

Are you aware that even after this PR, it seems that continue is still broken? i.e. it behaves like a break, and exits the enclosing for, instead of advancing its execution. For example, in:

package main

func inbreak() {
	println("BREAK")
	n := 2
	m := 2
	foo := true
OuterLoop:
	for i := 0; i < n; i++ {
		println("I: ", i)
		for j := 0; j < m; j++ {
			switch foo {
			case true:
				println(foo)
				break OuterLoop
			case false:
				println(foo)
			}
		}
	}
}

func incontinue() {
	println("CONTINUE")
	n := 2
	m := 2
	foo := true
OuterLoop:
	for i := 0; i < n; i++ {
		println("I: ", i)
		for j := 0; j < m; j++ {
			switch foo {
			case true:
				println(foo)
				continue OuterLoop
			case false:
				println(foo)
			}
		}
	}
}

func ingoto() {
	println("GOTO")
	m := 2
	foo := true
	i := 0
OuterLoop:
	i++
	for {
		if i > 5 {
			break
		}
		println("I: ", i)
		for j := 0; j < m; j++ {
			switch foo {
			case true:
				println(foo)
				goto OuterLoop
			case false:
				println(foo)
			}
		}
	}
}

func main() {
	inbreak()
	incontinue()
	ingoto()
}

// Output:
// BREAK
// I:  0
// true
// CONTINUE
// I:  0
// true
// I:  1
// true
// GOTO
// I:  1
// true
// I:  2
// true
// I:  3
// true
// I:  4
// true
// I:  5
// true

the behavior of incontinue() is not the same with Go and with Yaegi.

[mpl]

Same remark as above.

[mpl]

same remark as in break and continue.

[mpl]

If we are not in a forward case, it seems useless to do that, since the code in the goto case (L887) will overwrite c.tnext.

If we are in the forward case, we are overwriting c.tnext here. Is that what we want to do?