This patch makes it possible to annotate effectful operations at the
term-level. Type annotations are crucial when operations are
polymorphic. Prior to this patch, it was only possible to annotate
polymorphic operations in signatures. As a consequence it was
impossible to make the following kind of program type-check.

```links
handle(do Throw) {
  case ans -> Just(ans)
  case <Throw> : (forall a. () => a) -> Nothing
}
```

The problem is that Links does not infer polymorphism (rightly so), so
the inferred type of `Throw` will fail to check against the pattern
type in the `Throw`-case. With this patch we can annotate the invocation to make the program type check.

```links
handle(do (Throw : forall a. () => a)) {
  case ans -> Just(ans)
  case <Throw> : (forall a. () => a) -> Nothing
}
```