Various changes:

- 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.
Merge pull request #16 from greghendershott/string-ci

Change oneOfStrings result type, and provide case-insensitive alternatives for various
lookAhead returns parse result, but still consumes no input.

Note: This required upting `check-empty-parsing` in test-utils.rkt to
check for a result, just like `check-parsing`.  For parsers like $eof
the result is "".
Update doc for lookAhead.
extend lookAhead to return result; update docs and tests
Add `parse-result`. Closes #7.
Merge pull request #18 from greghendershott/parse-result

Add `parse-result`. Closes #7.