Add formatter for strict text

[dagit]

The first function I wrote to use text-format was this:

-- | A wrapper around `format` that returns a strict Text
sformat :: Params ps => Format -> ps -> Text
sformat f ps = toStrict (format f ps)

Perhaps that should be included in the library itself? (And the name format' might be more intuitive to folks).

[jsdw]

@dagit snap :)

[timbod7]

I wrote this as well.

[istathar]

I'd love it if the strict ones used ' as a suffix. That's by no means universal as a convention, but common enough that people would recognize it.