LinearMap: add `LinearMapView`, similar to `VecView`

[sosthene-nitrokey]

While implementing I realized that &mut LinearMap does not implement IntoIter. Should I add it ? 

I currently did not add it to LinearMapView either for symmetry, but that can lead to confusion. For example, the following doesn't compile:

use heapless::{LinearMap, LinearMapView};

let mut map: LinearMap<_, _, 8> = LinearMap::new();
let map_view: &mut LinearMapView<_, _> = &mut map;

for (key, val) in map_view {
    println!("key: {} val: {}", key, val);
}

The fix is to use instead

for (key, val) in &*map_view {

[sosthene-nitrokey]

replaced by #491