This PR introduces a no-op version of `Shrink`, a type that should allow
shrinking small types into smaller universes given a proof that the type
is small enough, and uses it in the iterator library. Because this type
would require special compiler support, the current version is just a
wrapper around the inner type so that the wrapper is equivalent, but not
definitionally equivalent.
While `Shrink` is unable to shrink universes right now, but introducing
it now will allow us to generalize the universes in the iterator library
with fewer breaking changes as soon as an actual `Shrink` is possible.
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 a special empty iterator type. Although its behavior
can be emulated with a list iterator (for example), having a special
type has the advantage of being easier to optimize for the compiler.
