lr-wpan: Adds namespace lrwpan and prefix shortening

This MR adds the namespace lrwpan to the lr-wpan module.

As discussed in the maintainers meeting, the namespace is not included in the helpers. Questions, comments, and corrections are welcome.

Edit: This MR also removes the prefix LrWpan from enumerations, structs, and some variables in the PHY and MAC.

Changes.md and Release notes will be updated before the final merge.