Thanks for the good work! I suggest that 'string-limit' be replaced with two separate functions, 'string-prefix' and 'string-suffix', because these are two different operations and it makes no sense to multiplex them onto a single function with an argument saying which one we are going to use.

Suggested patch attached. The names can be discussed; for example, 'string-left' and 'string-right' would do just as well.

If someone prefers saturating semantics for negative length arguments, then that's an option too. Ie, in case we want

 (string-prefix "abc" -2)

to yield the empty string instead of an error. I think an error is more useful.

The new functions are now inline (defsubst) in keeping with the spirit of subr-x.el. (We really should inline these better!)