Add pattern SString

[Bodigrim]

I wonder if this could provide a lightweight resolution to #207, alternative to #208. @newhoggy what do you think?

The idea is to use SString to perform T.pack / T.unpack when on GHC 7.10+ (which covers the vast majority of users) and resort to slow read . show (#207 (comment)) only on older systems.

[Bodigrim]

Actually, the latest incarnation of #208 is pretty lightweight, and this patch is obnoviously CPP-heavy, so dunno if I can claim any improvement ;) Feel free to close, this is just a raw experiment.

[sjakobi]

I don't have any personal experience with using -f-text, but it seems to me that the API enhancements in #208 are more powerful since they also provide ways for using the lazy-Text and Builder based parts of the prettyprinterAPI.

Nevertheless, I'm curious what @newhoggy thinks about this PR.

[Bodigrim]

@sjakobi if you are fine with #208, then it's indeed a more flexible and powerful way forward.