This is a performance optimisation for subtyping between two unions that
are largely the same.

Fixes #14034

This makes @adriangb's example in
#14034 (comment)
finish basically instantly. I could add it as a unit test?

Type checking pydantic core is still not fast — takes like four or five
minutes with uncompiled mypy — but at least it's now feasible. I think
there's room for doing some optimisation in make_simplified_union that
would improve this.

The following code optimises make_simplified_union in the common case
that there are exact duplicates in the union. In this regard, this is
similar to python#15104

To get this to work, I needed to use partial tuple fallbacks in a couple
places (these maybe had the potential to be latent crashes anyway?)
There were some interesting things going on with recursive type aliases
and type state assumptions

This is about a 25% speedup on the pydantic codebase and about a 2%
speedup on self check (measured with uncompiled mypy)

Fixes #15192

The following code optimises make_simplified_union in the common case
that there are exact duplicates in the union. In this regard, this is
similar to #15104

There's a behaviour change in one unit test. I think it's good? We'll
see what mypy_primer has to say.

To get this to work, I needed to use partial tuple fallbacks in a couple
places. These could cause crashes anyway.
There were some interesting things going on with recursive type aliases
and type state assumptions

This is about a 25% speedup on the pydantic codebase and about a 2%
speedup on self check (measured with uncompiled mypy)