internal/stack: helper package for querying stack frames

Intended for #5. Also replaces a helper function in the `printer`
package.

internal/stack/stack_test.go

@@ -0,0 +1,34 @@
+package stack_test
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/mmcloughlin/avo/internal/stack"
+)
+
+const pkg = "github.com/mmcloughlin/avo/internal/stack_test"
+
+func TestFramesFirst(t *testing.T) {
+	fs := stack.Frames(0, 1)
+	if len(fs) == 0 {
+		t.Fatalf("empty slice")
+	}
+	got := fs[0].Function
+	expect := pkg + ".TestFramesFirst"
+	if got != expect {
+		t.Fatalf("bad function name %s; expect %s", got, expect)
+	}
+}
+
+func TestMatchFirst(t *testing.T) {
+	first := stack.Match(0, func(_ runtime.Frame) bool { return true })
+	if first == nil {
+		t.Fatalf("nil match")
+	}
+	got := first.Function
+	expect := pkg + ".TestMatchFirst"
+	if got != expect {
+		t.Fatalf("bad function name %s; expect %s", got, expect)
+	}
+}