This results in windows users not needing to rename the DLL. This allows native llama builds to be dropped in, even on windows.

I also took the time to update the documentation, removing references to renaming the files, since the names now match.

Fixes #463

- Made `NativeApi` into a `static class` (it's not intended to be instantiated)
 - Moved `LLamaTokenType` enum out into a separate file
 - Made `LLamaSeqId` and `LLamaPos` into `record struct`, convenient to have equality etc

- using `Lazy<T>` to initialize it automatically.
 - Added in `AVX512` support for all dotnet versions (but not autodetected).
 - Added in AVX version auto detection.