Consider rewriting "interpolated strings" as "interpolated string literals"

Once #12 has been completed in terms of everything else, we should take another pass to see whether we can make sure we're always clear between the idea of an interpolated string literal (which is a purely source code concern) with "strings" (which exist at execution time). This may prove to be too awkward (and doing it while trying to get everything else right would definitely be counterproductive) but it's worth a try.