Support sorted merge join of vortex files/layouts efficiently

[joseph-isaacs]

We should be able to run sorted merge joins over vortex files without needing to loads all column into vortex arrays and without re-compressing

This involves the following parts:

• Support non-split-aligned scans. Instead of a scan returning a single ArrayStream it can return a Vec (element for each column) of array streams where each stream might be unaligned with the others.
• In the layout writer support writing unaligned columns/subtypes out of order. The number of element written to each column may different until the end of the file.

[joseph-isaacs]

Could these streams be segments from a flat layout or even a more complex layout such as a dictionary.