ref: More compact repr of projectkey

[untitaker]

We should be able to represent all DSNs with half the bytes. I validated
that we don't have non-hex DSNs in prod, this returns 0:

select count(*) from sentry_projectkey where encode(decode(public_key, 'hex'), 'hex') != public_key

This should solve the concern voiced by Jan in #795

#skip-changelog

[jjbayer]

Looks good. Tests are failing because as_str() is now missing, I wonder if we should expose the u128 value so we can do something like

assert_eq!(meta.public_key().0, 0xe12d836b15bb49d7bbf99e64295d995b);

in tests.