Remove Cow from type_row?

[aborgna-q]

Now that most optypes cannot be const-constructed anymore (see #1723), we are almost always constructing type rows from vectors.
The only cases where we use the const-constructed type_row! macro now is for empty typerows or rarely when using type_row![Type::UNIT].

The extra overhead of the Cow is probably not worth it anymore. We should try wrapping a Vec directly instead (and benchmarking the changes!).

[doug-q]

sync::Weak::new is const, but I guess we construct everything pointing to the extension nicely.