Generate a balanced tree with reduceTree (#2318)

The difference in logic depth for various paths now has a maximum of 1.

Also make treeReduce order the same for 2.12 and 2.13

.grouped(_) returns an Iterator
.toSeq on an Iterator returns a Stream in 2.12 and a List in 2.13
This can lead to changes in order when bumping from 2.12 to 2.13 that
can be avoided by simply using an eager collection explicitly.

Co-authored-by: Jack Koenig <koenig@sifive.com>