Implement better join and meet for protocols

[ilevkivskyi]

Currently, unrelated protocols give a non-trivial join only if one is a subtype of another. This have several disadvantages. One of the main is the lack of associativity (ditto for meet).

Several ideas for better join were proposed in #3132 (comment) and #3132 (comment)

This is a follow-up issue for #3132

[ilevkivskyi]

Related issue #3257

[ilevkivskyi]

#7932 has an important example for this.

[JukkaL]

Closing this since the potential fix looks quite complicated, and the cases where it would help are relatively rare.