fix #16752: threadvar now works with importcpp types; osx now uses native TLS (--tlsEmulation:off), which can be orders of magnitude faster (#16750)

* osx now uses native TLS, which can be orders of magnitude faster

* add {.cppNonPod.}

* improve test

* changelog, docs, disable part of windows test

tests/benchmarks/readme.md

@@ -0,0 +1,5 @@
+# Collection of benchmarks
+
+In future work, benchmarks can be added to CI, but for now we provide benchmarks that can be run locally.
+
+See RFC: https://github.com/timotheecour/Nim/issues/425