Add API documentation comments

[RadicalZephyr]

Most of the documentation was taken directly from Appendix A of the "Functional Reactive Programming" book and slightly adjusted to match the Rust semantics and documentation conventions.

Since the C++ API documented in that appendix is missing a few things present in the Rust API I had to write the documentation for those items from scratch. Extra scrutiny to those items would be appreciated to ensure their correctness. This includes:

• All SodiumCtx docs
• All Router docs
• The CellSink::cell method docs
• The StreamSink::stream method docs

I'm particularly vague on exactly what Router does, not having used it personally, though the type signature does seem to describe it fairly well.

In addition I hid the documentation for several things that are only incidentally public and not actually intended to be used by library consumers.

This is just a first pass of documentation for the crate. I intend to come back and add more exposition on the crate root and the documentation on each type, eventually including example code.

[RadicalZephyr]

And a belated Merry Christmas to you! 🎄

[clinuxrulz]

That looks great to me. Thank you.

The purpose of Router is to provide a more efficient Stream::filter for when your filtering a lot off the same stream. It creates less work for the backend when updating the graph. It is a last resort when struggling with performance.