Allow stri_locate_first's output to be consistent with that of stri_locate_all

Add `?simplify?` argument (`default=TRUE` for backward compatibility) to `stri_locate_first` and other `*_first` and `*_last` functions so that they generate the same kind of results as `*_all*`, i.e., lists.

Start with the stri_locate_first entry point and identify the related *_first and *_last functions named in the issue. Compare their current results with the corresponding *_all* functions; done means each accepts simplify=TRUE by default while supporting list-shaped results when simplification is disabled.

Written by the indexing model from the issue text.