avoid a check to is_char_boundary in split_at_position* for &str
avoid a check to is_char_boundary in split_at_position* for &str

the call to find() returns an index in the slice that is already at a
char boundary, so the check done by split_at is not needed. The
get_unhecked call is needed instead of self.get() to prevent the
reintroduction of bound checks

Geal/nom