Update unittests as disabled default construction is now fixed.
Add `unstd.casts` module.
Add classes casting functions.

Adds `unstd.casts.{tryDynamicCast,dynamicCastable,dynamicCast,tryUpCast,upCastable, upCast,downCast}` functions.
Use `heap` and `threadHeap` instead of `cHeap`.

`heap` is used in `WeakReference` and `WeakReferenceArray`.
`threadHeap` is used in `tempCString`.
[whitespace] Replace indention spaces with tabs.
Optimize `tempCString` for small strings.

Make it use stack allocated buffer for small strings.
Also add implementation note.