Speed up `nearest_common_ancestor()`.

Speed up `nearest_common_ancestor()`.

`nearest_common_ancestor()` uses an algorithm that requires computing
the full scope chain for both scopes, which is expensive because each
element involves a hash table lookup, and then looking for a common
tail.

This patch changes `nearest_common_ancestor()` to use a different
algorithm, which starts at the given scopes and works outwards (i.e. up
the scope tree) until a common ancestor is found. This is much faster
because in most cases the common ancestor is found well before the end
of the scope chains. Also, the use of a SmallVec avoids the need for any
allocation most of the time.