ENH: Cached cropped_bounds within regrid

See https://github.com/SciTools/iris/pull/1431 for further details.

Going from 30second (21600, 43200) source to N96 (145, 192):
Unoptimised: 43s
Optimised: 26s (does not include #1431)

Combined with #1431, offers a double in performance of regrid for the above.

When regriding from a 15seconds field to N2048 (decomposed to 30 pieces on multiple hosts using MPI) the combined optimisation of these PRs takes this from 20minutes to 3.7minutes.
The indices (or slice objects) are for the data array not the src_x_bounds array. I'd love a clarification in the docs.