Add withCString

[dpwiz]

Like #254, but only adds one function. peekCString is basically fromPtr0.

[dpwiz]

So, what's would be the next step in this?

I can remove the I8 argument, bringing the function in line with GHC withCString. Users can get text length for free with (+ 1) . lengthWord8.
withCStringLenAndNul can be judged separately later.

[Lysxia]

Users can get text length for free with (+ 1) . lengthWord8.

I think that's worth mentioning in the doc.

Otherwise, LGTM. For reference, this is essentially the same logic as Data.ByteString.useAsCString.

[Mikolaj]

So is Data.Text.Foreign.useAsCString planned or is withCString close enough (I haven't yet wrapped my head around it, but I need to decide whether to wait for text 2.1 with my update of sdl2-ttf to text2 or just use withCString)?

[Bodigrim]

@Mikolaj withCString is useAsCString in disguise.

[Mikolaj]

Indeed, it's precisely what I needed and it works fine: haskell-game/sdl2-ttf#16