Rename `FromDNSNames` to `NewDNSSlice`, since it's exactly `NewDNS`
except for slices.
Rename `AsProto` to use the "To" prefix, since it's the opposite of
"From".
Add a named type `ACMEIdentifiers` so that we can add methods to slices.
We will have a lot of slice handling code coming up, which this will
make more elegant and readable.
Add a comment to explain naming conventions in the `identifier` package.
Part of #7311
Alternative to #8068
9f4b18c6ce