[ty] Use a SmallVec for seen type aliases (#26187)
## Summary
Use a `SmallVec` for type aliases visited while checking
self-referential type-variable defaults.
Running ty over the entire ecosystem corpus shows this distribution
Seen aliases | Checks | Percentage
-- | -- | --
0 | 1,728,178 | 99.868647%
1 | 2,051 | 0.118524%
2 | 122 | 0.007050%
3 | 68 | 0.003930%
4 | 29 | 0.001676%
6 | 3 | 0.000173%
A `SmallVec` with an inline capacity avoids allocations in 99%, and the
linear scan is a non-brainer for up to 6 elements. There's a risk that
larger `seen` result in `O(n)` scans at every level, which would justify
a hash map. I'd argue that this isn't an issue in practice and we also
accept `O(n)` for our recursive Type visitor.
I'd say we use a `SmallVec`, we can always use a `SmallSet` if this ever
shows up on a real world project.
## Performance
Consistent 2-3% improvement across all projects
## Test Plan
Testing: Ran focused mdtests, repository hooks, and the 162-project
ecosystem corpus.
