feat(corelib): iter::adapters::Map

[julio4]

This is mostly an exploratory PR to test implementation of iterator adapters, i.e. functions that take an Iterator to return another Iterator. It's really useful as any type that implement IntoIterator can benefits from these adapters, and it also enables function chaining on iterators.

Added a first implementation of iter::adapters::Map:

let mut i = 1;
for elem in (1..4_u8).into_iter().map(|x| 2 * x).map(|x| x + 1) {
    assert_eq!(elem, i * 2 + 1);
    i += 1;
}

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I'm curious to know if an implementation of ops::FnMut would be possible, to be able to perform function calls with a reference? With it I believe the efficiency of these adapters can be greatly improved to avoid excessive copy/drop.

Feedback welcome on both the current implementation and potential optimization

[reviewable-StarkWare]

This change is