Instead of using enum variants to represent `kml:Link` and `kml:Icon`,
which makes it awkward to use them since enum variants are not
first-class types in Rust, they are implemented using two separate
structs.

Since the library already exports the public type `Kml::Icon` from the
`crate::types::style` module, `Kml:LinkTypeLink` and `Kml:LinkTypeIcon`
are exported. These are aliases for the internal types
`kml::types::link::{Link,Icon}`.

Enable and simplify reading these types using str's `parse` method.