Speed up signature verification in policy evaluation

[mtrmac]

After policy evaluation is merged (but filing now as a reminder to myself), signature verification in policy evaluation, with its temporary directory creation, will take about a second per case. Both general performance and test suite interactivity would greatly benefit from speeding this up.

At the very least we can reuse the temporary directories across all verifications with a single policy; that’s why PolicyContext exists in the first place. Also look into speeding up the verification as such, there might be a cheap optimization somewhere.