Proposed breaking change: apply-template to return a string instead of bytes

[otherjoel]

Currently the apply-template function provided by a #lang beeswax/template file returns bytes?. I’m considering changing it so it will return a string instead. An apply-template function provided by any other Racket module could still return bytes (or any type of value), but for Beeswax templates I think it makes more sense to return a string.