Add a skeleton for SrtLibrary, representing a shared library

The idea is that this is the higher-level API for what @denittis is doing in !2 (merged).

@denittis, if this makes sense to you, please rebase onto it and make your function call _srt_library_new() when appropriate. It would probably make sense to put the library-inspecting code in library.c rather than integrity.c too, since there's very little actual integrity checking so far.

/cc @jpwhiting - both of you having a look at this would be useful. The API I sketched out in https://phabricator.collabora.com/T15637 calls for a SrtGraphics object that is quite a lot like SrtLibrary, but different. Either I could knock together a skeleton for that tomorrow, or you could do it.

So far I haven't added an implementation of SrtSystemInfo, which was meant to do the work to create the appropriate objects on first call, and cache them until the SrtSystemInfo is destroyed (since their results won't change). At this early stage I think it's fine to break ABI, so we can add simple functions like the one @denittis added in !2 (merged), and then remove them in favour of SrtSystemInfo when it's ready.