* Fixed some small typos in api-users-guide.md

* Fix some small typos in slang-test/main.cpp, render-test/render-d3d11.cpp

* Remove exit() calls from test code. Added Slang::Result, which works in the same way as COM HRESULT.

* FIx bug introduced when moving to Slang::Result - handling E_INVALIDARG on Dx11.

* Fix the testing of feature levels on Dx11 renderer.

* First attempt at README.md for slang-test.

* Tidied up the slang-test README.md file.

* Fix some small typos in tools/slang-test/main.cpp

* Fix spaces -> tabs problems.
Fix some small types.

* Refactor Renderer implementations such that:
* Class definition does not contain long implementation/s
* Removed unused globals
* Ordered implementation after class definition
* Made renderer specific classes child classes, and use Impl postfix to differentiate
* Converted tabs into spaces

* First pass at Slang::ComPtr. Added slang-defines.h which sets up some fairly commonly used defines such as SLANG_FORCE_INLINE, compiler detection, os detection, and some other cross platform features.

* * Fixed bug in vk renderer - where features structure not initialized on hkCreateDevice
* Make member variables in Renderer implementations use prefix
* Updated test README.md to document that free parameter can control what test is run

* * changed setClearColor to take an array of 4 floats to make API clearer on usage
* mix of type usage style - defaulted to more conventional style

* * Fixed swapWith
* Use SLANG_FORCE_INLINE
* Don't bother initializing List data when type is POD

* Added convenience macro for Result handling SLANG_RETURN_NULL_ON_ERROR