Columns should be backed by a BTree, not an array

Follow up to 8099.

We have pretty terrible lookup performance as the number of columns grows (linear). In at least one location, this results in quadratic performance.

We don't however want this structure to be either any more expensive to build, nor to store. Some small modifications to BTree will permit it to serve here, by permitting efficient lookup by index, and calculation of index for a given key.