bounds checking of "abc"[1:3] is very redundant

[StefanKarpinski]

Partly due to #24840, taking a slice of a string ends up checking bounds at least three different times. This should be reduced to a single time.

[laborg]

On master I could only spot one boundcheck (and #29796 has improved performance and added the necessary @inline).

[laborg]

Ah, calling j = nextind(s,j) -1 might still be a redundant.