Change TracksInfo::tracks to return an iterator

Using an iterator instead of a static slice allows for more flexible
implementations of `TracksInfo` that can use the chain storage without
compromising a lot on the performance/memory penalty if we were to return
an owned `Vec` instead.

NOTE: This feature will benefit from the soon to be released
return_position_impl_trait_in_trait(https://github.com/rust-lang/rust/pull/115822)
to not require the `TracksIter` associated type and from a bugfix in the
compiler(https://github.com/rust-lang/rust/issues/116662) to declare the
DEFAULT_MAX_TRACK_NAME_LEN generic constant in the `TracksInfo` trait.