Simplify some type switchy code
Renaming for accuracy
Re-enable row collapsing and add an insertion benchmark

With a 1M row table, insertions were taking about 3.5ms/row on my
machine (< 300qps), and it's n^2 in table size.
Make StringColumnsMap public

It was the sole private field (out of 17) for no obvious reason.

We should do a real public/private pass later on.