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.
This PR provides the means to reason about "equivalent" iterators.
Simply speaking, two iterators are equivalent if they behave the same as
long as consumers do not introspect their states.
This PR provides the iterator combinators `takeWhile` (forwarding all
emitted values of another iterator until a predicate becomes false)
`dropWhile` (dropping values until some predicate on these values
becomes false, then forwarding all the others).
This PR provides the iterator combinator `filterMap` in a pure and
monadic version and specializations `map` and `filter`. This new
combinator allows to apply a function to the emitted values of a stream
while filtering out certain elements.
`map` should have an optimized `IteratorCollect` implementation but it
turns out that this is not possible without a major refactor of
`IteratorCollect`: `toArrayMapped` requires a proof that the iterator is
finite. If `it.mapM f` is `Finite` but `it` is not, then such a proof
does not exist. `IteratorCollect` needs to take a proof that the loop
will terminate for the given monadic function `f` instead. This will not
be done in this PR.
