add 'isAscii' and 'isSymbol' from Haskell's Data.Char

[cdepillabout]

I'd like to have some functions from Haskell's Data.Char available, like isAscii and isSymbol.

Would a PR adding these be accepted?

[paf31]

👍

[cdepillabout]

So this ended up being much more difficult than I originally imagined.

I made a PR at #57.

[cdepillabout]

This has been released as the purescript-unicode package.