remove bivariance

There is one fishy part of these changes: when computing the LUB/GLB of
a "bivariant" type parameter, I currently return the `a`
value. Bivariant type parameters are only allowed in a very particular
situation, where the type parameter is only used as an associated type
output, like this:

```rust
pub struct Foo<A, B>
    where A: Fn() -> B
    {
        data: A
        }
        ```

In principle, if one had `T=Foo<A, &'a u32>` and `U=Foo<A, &'b u32>`
and (e.g.) `A: for<'a> Fn() -> &'a u32`, then I think that computing the
LUB of `T` and `U` might do the wrong thing. Probably the right behavior
is just to create a fresh type variable. However, that particular
example would not compile (because the where-clause is illegal; `'a`
does not appear in any input type). I was not able to make an example
that *would* compile and demonstrate this shortcoming, and handling the
LUB/GLB was mildly inconvenient, so I left it as is. I am considering
whether to revisit this.

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