Fix iterator lifetimes

[aleokdev]

This PR fixes some small lifetime issues with moderately big implications. Currently, code like this fails:

use tiled::Object;

let spawnpoints: Vec<Object> = map
    .layers()
    .filter_map(|layer| match layer.layer_type() {
        tiled::LayerType::ObjectLayer(layer) => Some(layer),
        _ => None,
    })
    .flat_map(|layer| layer.objects())
    .filter(|object| object.obj_type == "spawn")
    .collect();

dbg!(spawnpoints);

The compiler complains with:

error[E0515]: cannot return reference to function parameter `layer`
  --> src/layers/object.rs:88:23
   |
17 |     .flat_map(|layer| layer.objects())
   |                       ^^^^^^^^^^^^^^^ returns a reference to data owned by the current function

error: aborting due to previous error

This is due to the declaration of impl ExactSizeIterator functions:

pub fn objects(&self) -> impl ExactSizeIterator<Item = Object>

Which, expanded, look like this:

pub fn objects<'s>(&'s self) -> impl ExactSizeIterator<Item = Object<'s>> + 's

Which limit the lifetime of the iterator and its items to the mapwrapper object. This is not required because the mapwrapper object is just a reference tuple, and so these functions should be like this:

pub fn objects(&self) -> impl ExactSizeIterator<Item = Object<'map>> + 'map

This PR fixes these issues and enforces the fix with some doctests.