Change oneOfStrings result type, and provide case-insensitive alternatives for various

[greghendershott]

• Change oneOfStrings to return (listof char?) not string?
• Add charAnyCase. Like char, but case insensitive.
• Add stringAnyCase. Like string, but case insensitive.
• Add oneOfStringsAnyCase. Like oneOfStrings, but case insensitive.
• Refactor oneOf and noneOf to share common code.
• Fix doc of Pos field names. While I'm at it, change the first field (previously called pos in the docs but x in the code) to be named ofs, which I think is clearest. Errors are often reported in terms of line, column, and absolute offset. So I think ofs is a good name.

NOTE: Even though I'm a collaborator on this repo now (thanks again!) these were enough changes, and we'd only really discussed one of them, that I felt better doing this as a pull request so you could review (if you want).

[stchang]

Changes look great. Thanks!