IntoEnumIterator should have a constraint on Iterator: Iterator<Item=Self>

[Ten0]

IntoEnumIterator does not have a constraint on Iterator: Iterator<Item=Self>.
When writing generic code on any IntoEnumIterator, it forces us to write the very verbose:

where
	<Self as IntoEnumIterator>::Iterator: Iterator<Item = Self>,

though that behaviour could be enforced as it is documented on the trait.

We could avoid having to write this by adding an Iterator<Item=Self> bound on the associated type Iterator of IntoEnumIterator.

[Peternator7]

Simple and shouldn't break anyone unless they were manually implementing the trait on something other than an enum.

This was easy so I went ahead and added it to version 0.18.0. [https://crates.io/crates/strum]