[SPARK-15742][SQL] Reduce temp collections allocations in TreeNode transform methods

In Catalyst's TreeNode transform methods we end up calling `productIterator.map(...).toArray` in a number of places, which is slightly inefficient because it needs to allocate an `ArrayBuilder` and grow a temporary array. Since we already know the size of the final output (`productArity`), we can simply allocate an array up-front and use a while loop to consume the iterator and populate the array.

For most workloads, this performance difference is negligible but it does make a measurable difference in optimizer performance for queries that operate over very wide schemas (such as the benchmark queries in #13456).

### Perf results (from #13456 benchmarks)

**Before**

```
Java HotSpot(TM) 64-Bit Server VM 1.8.0_66-b17 on Mac OS X 10.10.5
Intel(R) Core(TM) i7-4960HQ CPU  2.60GHz

parsing large select:                    Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
1 select expressions                            19 /   22          0.0    19119858.0       1.0X
10 select expressions                           23 /   25          0.0    23208774.0       0.8X
100 select expressions                          55 /   73          0.0    54768402.0       0.3X
1000 select expressions                        229 /  259          0.0   228606373.0       0.1X
2500 select expressions                        530 /  554          0.0   529938178.0       0.0X
```

**After**

```
parsing large select:                    Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
1 select expressions                            15 /   21          0.0    14978203.0       1.0X
10 select expressions                           22 /   27          0.0    22492262.0       0.7X
100 select expressions                          48 /   64          0.0    48449834.0       0.3X
1000 select expressions                        189 /  208          0.0   189346428.0       0.1X
2500 select expressions                        429 /  449          0.0   428943897.0       0.0X
```

###

Author: Josh Rosen <joshrosen@databricks.com>

Closes #13484 from JoshRosen/treenode-productiterator-map.