Update to heapless 7.0

[diseraluca]

This project uses heapless for its queue implementation. The currently pinned version is 6.0.

As a result of the stabilization of min_const_generics in rust 1.51, most crates that were depending on typenum are updating to const generic parameters.

For heapless, the 6.0 -> 7.0 update does so.

The crate already makes use of const generics, previously trough nightly, so that updating would further improve the consistency of the code.

The update should be mostly painless, but will require updates in many places, as almost every type in rx and tx makes use of generically bound heapless types.

The update path, at the minimum, will require:

• The removal of any reference to heapless::consts.
• The removal of any reference to a typenum-like type.
• The update of each typenum-like generic parameter to a const generic usize parameter.
• The update of the pinned heapless version in the manifest file.