Fixes for ranges, slice support

I cherry-picked the non-specific things (hence the mixing in of a few unrelated small fixes) from https://gitlab.cern.ch/cp3-cms/bamboo/merge_requests/26 (which needs a rebase and some adjustment after this), and supported slices for all ranges.

The simple cases I tested seem to work fine (slice of original collection, selection, on-the-fly calculated variation, and slice-of-slice), but I might have missed some corner cases, so more test cases are definitely welcome. I put a bit of optimisation not to insert 0+i in case a slice starts at the beginning - mostly to simplify the intermediate expressions; otherwise they would be eliminated at compile time.