hex colors

[Terrel Shumway]

https://gist.github.com/terrelcodes/2717f5661e24c19794dbe03fa276a0d3

I was surprised that there isn't a standard way to use RGB hex codes as color names. I was equally surprised to find that string-to-number doesn't handle hex digits.

So I wrote the functions that I thought were missing.

I'm not sure how to best handle invalid digits—maybe return Option<Number>?

Is this worth adding to Pyret or Racket?