For performance reason, the contract equality checker performs a quick
pointer check to see if the two terms to compare are physically equal.
In this case, it used to return `true` directly, eschewing more costly
recursive checks.

However, this is unsound, because the same term (physically) might be in two
different environments, and have two different values. This is typically
the case with function application: when evaluating `f 1` and `f 2`,
both terms will physically point to the body of `f`, but in a different
environment. Thus, we also need to ensure that environment are equals as
well in this quick check.

This commit adds a fast `ptr_eq` check to the environment as well, and
now checks that both terms and their respective environments are
pairwise physically equals.

Passing by, this commits also fix unrelated clippy warning, after the
udpate to clippy 1.73.