Support pattern-matching in comprehensions

Should compile
```python
[a + b for Pair(a, b) in pairs]
```
or
```python
[a + b match for Pair(a, b) in pairs]
```
into the equivalent of
```python
[(def (Pair(a, b)) => a + b)(p) for p in pairs]
```

Start by tracing Coconut’s compiler handling for comprehensions and pattern matching, using the issue’s two proposed forms as the cases to investigate. Confirm that the implementation produces the shown equivalent behavior for Pair(a, b) over pairs, and add focused tests for the supported syntax and result.

Written by the indexing model from the issue text.