update string functions

[shriram]

Joe has pushed new string functions getting rid of the -1 return value. Update documentation and add docs for string-find and string-find-opt.

[shriram]

@jpolitz I can't find either of these on https://pyret-horizon.herokuapp.com/editor — is there some module I should be importing?

[shriram]

FYI — I can't document this properly until I can run it, and I haven't been able to get a running version. So right now (a) it's in the docs and (b) the docs are broken. If this won't go out Aug 26 I need to remove these; if they will, I need access sooner so I can fix the docs. Thanks. @jpolitz

[jpolitz]

Ugh sorry these are there on https://pyret-horizon.herokuapp.com/editor now (and deploying to extension, etc).

They've been “there” for a while (even with test cases in pyret-lang) and not in starter2024 so hard to get to in CPO.