Expose ShortByteString construstor

[Bodigrim]

After #305 there remain no reasons to hide SBS constructor into an Internal module: there is no internal invariant left which could possibly violated by its exposure.

This allows users to implement byteArrayToByteString and byteStringToByteArray from #186 without reaching out for internal modules. Cf. #253.

[sjakobi]

Is there any chance that we might want to change ShortByteString's internal representation?! In that case we could save ourselves some hassle by keeping the constructor internal.

With this change, we can save our users some hassle though, so 👍.

[Bodigrim]

Data.ByteString.Short.Internal is public (sigh), so changing the internal representation of ShortByteString would be a hassle anyways. However, I do not expect it happening any time soon: there is not much choice for unpinned, GC-managed memory other than ByteArray.