? stri_ranges_union(), stri_ranges_intersect(), stri_ranges_diff() ?

These functions may be useful when operating on results of stri_locate_*

e.g.
x <- stri_locate_all_charclass(str, "WHITESPACE")
y <- stri_locate_all_charclass(str, "L")
stri_sub(str, stri_ranges_union(x, y)) # extract whitespaces and letters

some options:
- operate row by row on a single matrix (stri_locate_first/last)
- operate on all rows on a matrix or a list of matrices (stri_locate_all)

these may be auto-detected, i.e. whether we have a list of matrices or a single matrix