Consider implementing lazy loading

[szhorvat]

LibraryFunctionLoad is slow enough to cause a noticeable delay with ~100 functions. On a Raspberry Pi it takes 30-60 s to load IGraphM.

Consider implementing optional lazy loading to improve loading performance.

It must not be hard-coded because it must be possible to detect loading errors at package loading time. This is essential for development. It should be possible to optionally enable it for release.