<small_vector>: Feasibility of certain operations on small_vector with the same T but different other parameters

Some operation, such as assigment, comparison and copy/move construction should be allowed.
```
small_vector a{1,2,3};
small_vector b{1,2,3};
a==b;
a=b;
...
```

Start by locating the small_vector implementation and its tests, then inspect how operations are constrained for differing template parameters. Compare assignment, comparison, and copy/move construction for small_vector and small_vector. Done means the supported behavior is defined and covered by tests, or the issue is documented as infeasible.

Written by the indexing model from the issue text.