Use compact string representations in interface files

Storing strings as [Char]s is very inefficient, especially given that each such instance of a string will be duplicated when read back in from a serialized interface file.

Storing them as FastStrings instead allows us to use a compact representation while also ensuring that when these strings are read back in, each unique string will only be allocated once.

Using ShortBytestring directly is also an alternative, but then we lose the deduplication.