Remove other functionality not needed in Rust

[jbangelo]

The final chunk of changes from #122 🎉

Simply put this removes the remaining functionality that depends on the C library and breaks the dependency on the swiftnav-sys crate. I wouldn't say that we'll never want this functionality in Rust, but these modules have not been meaningfully used up till now and have non-trivial C code backing them so would require an undue amount of work to re-write.

What's been changed

• Crate level documentation has been updated to not reference removed functionality, and to give a description of functionality originally missed
• Minimum supported Rust version (MSRV) has been bumped up to reflect reality (also fix CI build to check MSRV still builds)

What's been removed

• ephemeris module - Decoding and calculating Satellite positions
• geoid module - Calculating difference between Ellipsoid and Geoid
• ionosphere module - Decoding and evaluating Klobuchar ionosphere model
• troposphere module - Calculating troposphere delays
• navmeas module - Datatypes related to storing GNSS measurments
• solver module - Single epoch position solver
• All conversions to/from swiftnav_sys types, all of these were private

What's been added

• N/A

[JADC362]

Thanks Joseph for working on this!