Crate-ize Sparse21

[dan-fritchman]

We would like this repository to become the source for the sparse21 crate.

This would require:

• Pull sparse21/ up to “crate level”
• Bring along spnum module, make it a public export from the new sparse21 crate
• Add back all the TOML, config, docs, etc from the existing crate

Open question(s):

• The Result/ error type(s). Currently shared between sparse21 and the rest of spice21. Either:
  • sparse21 can export one and spice21 can use it, or
  • They can both define one, with conversions there-between