Add optional strum to enums, update README, add more wrappers

[joe-saronic]

The original purpose of this PR was to add strum::Display (and therefore ToString) support for enums in this crate. My particular usescase was that I wanted to dump the PPK fix status as a string rather than a number into an output file. Since then, it's grown to add more functionality:

• Bumped the underlying RTKLib to latest (some bugfixes, no major API changes)
• Added README as discussed in Update lib #1
• Added SBF parser wrappers
• Added better support for decoders, since a bunch of them have shared interface
• receivers feature flag for all the receiver types known to rtklib (TODO: AdvancedNavigation)
• convrnx wrapper under conv feature, which requires receivers
• Refactored some existing interfaces to set better precedent for future development

A design note: num_enum stays as-is since it's very lightweight and the TryFromPrimitive conversion is much nicer IMO, than what strum would offer with FromRepr.

Tested this by using the new modules in our code. Everything seems to work nominally.

[kpwebb]

This looks great -- I'm going to do some additional testing, but let me know if you're ready for me to review or if this is still WIP.

[joe-saronic]

This looks great -- I'm going to do some additional testing, but let me know if you're ready for me to review or if this is still WIP.

This is in active use on my end. I'm going to make more PRs as I go, but this one should be pretty stable. I appreciate the additional testing.

[joe-saronic]

I added one more commit bumping the syslib version since I had forgotten to do that, but aside from that everything still appears to be working. As an FYI, I currently have the following PR in the pipeline, which I will transfer over once the conditions are met: joe-saronic#1