Make str::as_buf less error prone.

[brson]

as_buf passes to the callback the length of the string + 1 for the null character. Now that it as_buf also accepts slices this results in some very confusing behavior (#3365).

I suggest we remove the len parameter completely. Users that care about the length should capture it and not assume anything about null termination (as_c_str is for null termination).

[graydon]

See also #7235 , I expect we'll be overhauling all of this.

[huonw]

Fixed by #8296.