Disallow keywords as identifiers in hints unless within backticks

For example it's currently possible to write hints such as

```
fun () ===> fun _
```

This confusingly checks for application of unit to an identifier `fun`. To avoid any confusion, keywords should only be allowed as identifiers when in back ticks, e.g. that hint should have to be:

```
``fun`` () ===> ``fun`` _
```
