with this, hopefully more obvious array accesses will be handled
automatically.
Just like #3503, this PR does not investiate which of the exitsting
tactics in `get_elem_tactic_trivial` are subsumed now and could be
dropped without (too much) breakage.
The pattern
```
for h : i in [:xs.size] do
let x := xs[i]'h.2
```
is occassionally useful to iterate over an array with the index in
hand. This PR extends the `get_elem_tactic_trivial` so that one can
simply write
```
for h : i in [:xs.size] do
let x := xs[i]
```
fixes#3032.
