Add doctests for keys() and values() of the BTreeMap collection.

I couldn't get it to work without the annotations. I had to do the same in my update to #19488.

Without annotations:

```
<std macros>:6:23: 6:33 error: the type of this value must be known in this context
<std macros>:6                 if !((*given_val == *expected_val) &&
```