This PR removes the `Subarray`-specific `toArray`, `foldlM` and `foldl`
methods and instead provides these operations on `Std.Slice`, which are
implemented with the `ToIterator` instance of the slice. Calling
`subarray.toArray` etc. still works, since `Subarray` is an abbreviation
for `Slice _`.
Because the benchmarks are not so clear, to be safe, I will merge this
only after the release. In contrast to the ranges, the iteration over
slices is not quite as efficient as the old `Subarray`-specific
implementation, which would require either more optimizations in the
iterator library (special `IteratorLoop` and `IteratorCollect`
implementations) or better unboxing support by the compiler.
This PR adjusts the experimental module system to make `private` the
default visibility modifier in `module`s, introducing `public` as a new
modifier instead. `public section` can be used to revert the default for
an entire section, though this is more intended to ease gradual adoption
of the new semantics such as in `Init` (and soon `Std`) where they
should be replaced by a future decl-by-decl re-review of visibilities.
This PR moves parts of the iterator library from `Std` to `Init`. The
reason is that the polymorphic range API must be in `Init` and it
depends on the iterators.
