Implement commonly used combinations of Iterator::map

Examples

# use itermaps::MapExt;
let arr = [[1, 2], [3, 4]];
let first: Vec<i32> = arr.iter().map_index(0).copied().collect();
assert_eq!(first, [1, 3]);

let arr = ["foo", "bar"];
let arr1: Vec<String> = arr.into_iter().map_to_owned().collect();
assert_eq!(arr1, arr);