Basic iterator support

[udoprog]

This is just a POC for adding iterator support to Rune.

The Iterator type is a type-erased implementation of all various forms of iterators that are supported.

This allows for the following:

let values = [1, 2, 3];
values.iter().map(|n| n + 1).filter(|n| n > 2).collect_vec()

This adds support for:

• size_hint - To return the expected size of the iterator.
• map - To map an iterator.
• filter - To filter an iterator.
• rev - To reverse an iterator, which is only supported for double-ended iterators. Else it will panic.
• chain - To chain two iterators.
• enumerate - To create an enumerating iterator, which is only supported by exact-size iterators. This is detected if the lower and upper bound of size_hint agrees. Object iterators specifically doesn't have a known size (Same as the Rust impl, but dynamically).
• Coercion into an iterator for Vec, Object, and Option.