There are many more lemmas about `foldlM`, so this may be useful for
reasoning about for loops by transforming them into folds.
The transformation includes accounting for monad effects, but does have
a mild performance difference in that short-circuiting on
`ForInStep.done` is replaced by traversing the rest of the list with a
noop.
I'd previously added an instance from `ForIn'` to `ForIn`, but this then
caused some non-defeq duplication. It seems fine to just remove the
concrete `ForIn` instances in cases where the `ForIn'` instance exists
too. We can even remove a number of type-specific lemmas in favour of
the general ones.
We make sure that we can pull `List.toArray` out through all operations
(well, for now "most" rather than "all"). As we also push `Array.toList`
inwards, this hopefully has the effect of them cancelling as they meet,
and `simp` naturally rewriting Array operations into List operations
wherever possible.
This is not at all complete yet.
