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

.golangci.toml

@@ -60,7 +60,7 @@ enable = [
     # "nlreturn",
     "noctx",
     "nolintlint",
-    "paralleltest",
+    #"paralleltest",
     "prealloc",
     "predeclared",
     "revive",