Stack-based unmarshaler (#546)

* Benchmark script

* Rewrite unmarshaler using the stack

Instead of tracking the build chain using `target`s, use the stack
instead.

Working and most benchmarks look good, but regression on structs unmarshalling.

~60% slower on ReferenceFile/struct.

* Shortcut to check if last node of iterator

* Remove unecessary pointer allocation

* Skip over unused keys without marking them as seen

* Add some tests

* Fix mktemp on macos

internal/ast/ast.go

@@ -28,6 +28,12 @@ func (c *Iterator) Next() bool {
 	return c.node.Valid()
 }
 
+// IsLast returns true if the current node of the iterator is the last one.
+// Subsequent call to Next() will return false.
+func (c *Iterator) IsLast() bool {
+	return c.node.next <= 0
+}
+
 // Node returns a copy of the node pointed at by the iterator.
 func (c *Iterator) Node() Node {
 	return c.node