Prepare release 0.13.0

[lqd]

This PR prepares a polonius-engine release:

• removes the warnings for the 2 currently unused context sharing fields for the Hybrid variant: the potential errors that the LocationInsensitive variant computes can (and will, in the future) be used by the Naive/DatafrogOpt variants to limit their own computation to the loans/subsets that actually matter.
• updates the version to 0.13.0, which includes the results of the more recent round of renaming, and changes to compute subset errors in all variants: this will allow the Hybrid variant to be the default once again.

[lqd]

Testing this branch under rustc looks good to me so I'll merge and do the release. Then I'll open a PR to update rustc to the released version.