Add mention of single-quote character literals.

[jemc]

As far as I can tell, single-quote character literals are not documented in the tutorial anywhere. This would have prevented some confusion reported in ponylang/ponyc#756.

[SeanTAllen]

Any thoughts on where in the current tutorial structure to put this @jemc?

[jmiven]

I was wondering if Pony had character literals when I found this issue, so my beginner-level input might be helpful.

I would expect to find such information in the Expressions chapter, near the Variables section. I also looked in the "Primitives" section (chapter 2) though, because I remembered that it mentioned integers.

[SeanTAllen]

@jemc is this still not documented?

[SeanTAllen]

Does #250 address this?

[jemc]

I don't think #250 addresses it, because it didn't add anything related to character literals like 'A'.

[mfelsche]

With #223 being merged these are documented.