Change HirVec<P<T>> to HirVec<T> in Expr.

Change HirVec<P<T>> to HirVec<T> in Expr.

This changes structures like this:
```
[ ExprArray | 8 | P ]
                  |
                  v
                  [ P | P | P | P | P | P | P | P ]
                    |
                    v
                    [ ExprTup | 2 | P ]
                                    |
                                    v
                                    [ P | P ]
                                      |
                                      v
                                      [ Expr ]
```
to this:
```
[ ExprArray | 8 | P ]
                  |
                  v
                  [ [ ExprTup | 2 | P ] | ... ]
                                    |
                                    v
                                    [ Expr | Expr ]
```