Optimize diff cmd (horrendous memory usage)

[neilotoole]

Is your feature request related to a problem? Please describe.
sq diff uses a shocking amount of memory. The docs describe sq diff as being in beta for this reason. It's already noted in the source code that diff needs to be optimized.

It's pretty bad (memory usage circled in magenta):

Describe the solution you'd like
There are massive optimizations that can be made to the diff code. Some preliminary work is already in the source code.

[neilotoole]

Release v0.48.1 tackles a bunch of the diff performance issues. What was once 22GB of ram is now more like 22MB. But there's still plenty more low-hanging fruit for diff perf.