lr-wpan: Change SuperframeField to standard bitmap

This MR is a follow-up to !1686 (merged). It changes the use of the MAC superframe to a bitmap (as described by the standard). This reduces some complexity in the MAC internal management of this field (For instance, it removes the need of specialized Serialize and Deserialize functions for this field).

This MR will be followed by 1 or 2 other MR which eventually will remove the dependency of lr-wpan-pl-headers.h on lr-wpan-fields.h.

RELEASE NOTES will be changed before the final merge.