Check the `NoThunks` invariant during a nightly CI test

If you set the following flag and run the test suite, most `TVar`s -- including all critical ones, unless we have a bug -- will check their contents for thunks via the `nothunks` library whenever they're updated.

```
$ cat cabal.project.local
package io-classes
flags: +checktvarinvariant
```

This flag makes the tests run quite slow, so it's something we had been doing incrementally. But I, for example, haven't done it since taking over lead for two months! So adding it to some CI (nightly, but perhaps even just weekly) would be much safer.

Edit: `io-sim-classes` was renamed `io-classes` on 2021 July 8.