Use features to make some dependencies optional

[matt-cornell]

This is a big library that's able to do a lot of things, but for my usage, I only need the basic HEALPix operations. While one solution could be to split this into multiple crates, I made the less invasive change of adding feature flags for things that aren't necessary for the library's functioning. Since there's not a standard way of documenting feature flags, I haven't documented them, but I'd be happy to do so however you'd like.

The new features and dependencies they enable are:

• base64 (only enables base64, required for reading and writing compressed MOCs as base64)
• memmap (enables memmap2, required for all of the FITS file IO)
• skymap (enables colorous, mapproj, and png, required for image saving)
• serde (only enables serde, required for Serialize and Deserialize on stuff in cdshealpix::nested::map::mom::impls::ported_from_mom_builder)
• sort (enables bincode, toml, memmap, and serde, required for cdshealpix::nested::sort)

[fxpineau]

@matt-cornell
Thank you very much: it is something I also had in mind (to make features to lower the number of dependencies when only interested in core functionalities), and you did it 👍
As soon as I have time, I will have a look at the PR.