Update amalgamation file

Update amalgamation file

Update amalgamation file

Update amalgamation file

Update amalgamation file

Fix tests on luau

Add some additional tests

Update documentation (#233)

* Update Doxygen documentation.
* Add brief API reference to the manual.
* Update CHANGES.md for 2.6.

Introduce isInstance type checking function (#218)

* Introduce Stack<T>::isInstance() function
* Fix build error