Add methods to individually open standard libraries.

[Ale32bit]

This PR adds all individual methods to open the standard libraries:

• OpenBase (_G)
• OpenPackage
• OpenCoroutine
• OpenTable
• OpenIO
• OpenOS
• OpenString
• OpenMath
• OpenUTF8
• OpenDebug

I could not make it build on my machine (both before and after the addition), so it would be nice to have it verified by someone more capable than me.

[kelvincookshaw-unity3d]

I've started adding code like this to my project. It would be better for us if this was in KeraLua, and the project code didn't have to reference the native library. I tried to look at the errors for the failing checks, but I don't have access to the Azure organization in which the build happens.

What would it take to get this PR merged?

[viniciusjarina]

Why not simply p/Invoke the lua_openxxx functions ?

[thedudxo]

Why not simply p/Invoke the lua_openxxx functions ?

The entire point of using a wrapper library like this is so that everyone doesn't have to redefine the same p/invokes

[viniciusjarina]

My point is why not simple call luaopen_coroutine and other luaopen_* instead of using RequireF ? Just like OpenLibs. KeraLua is a map 1:1 to Lua API. if OpenCoroutine is doing more the simple invoking luaopen_coroutine