Release supporting agda-stdlib 2?

The currently released version fails to build with agda-stdlib 2.0. This is fixed in master, but that fails to build for a different reason:

```
/build/source/examples/Expr.agda:8,1-26
Importing module Level.Bounded using the --guardedness flag from a
module which does not.
when scope checking the declaration
  open import Level.Bounded
```