go/types, types2: simplify unifier

The unifier was written such that it was possible to specify
a different set of type parameters (declared by different
generic declarations) for each type x, y being unified,
to allow for what is called "bidirectional unification"
in the documentation (comments).

However, in the current implementation, this mechanism is
not used:

- For function type inference, we only consider the
type parameter list of the generic function (type parameters
that appear in the arguments are considered stand-alone types).
We use type parameter renaming to avoid any problems in case
of recursive generic calls that rely on type inference.

- For constraint type inference, the type parameters for the
types x and y (i.e., the type parameter and its constraint)
are the same and had to be explicitly set to be identical.

This CL removes the ability to set separate type parameter
lists. Instead a single type parameter list is used during
unification and is provided when we initialize a unifier.

As a consequence, we don't need to maintain the separate
tparamsList data structure: since we have a single list
of type parameters we can keep it directly in the unifier.

Adjust all the unifier code accordingly and update comments.

As an aside, remove the `exact` flag from the unifier as it
was never set. However, keep the functionality for now and
use a constant (exactUnification) instead. This makes it
easy to find the respectice code without incurring any cost.

Change-Id: I969ba6dbbed2d65d06ba4e20b97bdc362c806772
Reviewed-on: https://go-review.googlesource.com/c/go/+/463223
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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