Various bits of cleanup

[vyasr]

I was trying to run the benchmarks in the repo and found various small improvements that should help with being able to run these more out of the box:

• The Makefile now supports running all the benchmarks without the virtual env if desired.
• I removed the PYTHONPATH override, which prevents the various python -m commands from running since that relies on the current directory being on sys.path
• I added support for passing the python executable as an argument so that the no-env jobs can reuse bits if they need to. For now, we're not using that since the only run target that depends on building the tables is run-polars, but if we want to change that so that all targets rebuild the tables this change would help
• Changes the ifndef to an ifndef-else to remove warnings around multiple definitions of a target. Feel free to let me know if any of these changes seem undesirable for some reason that I am not aware of.

In the process I also improved docs/comments in a couple of places and fixed some formatting. I also deleted a file that looks like it was recently added accidentally in #144.

[vyasr]

Based on some feedback in the Discord, I'll attempt to split this PR up into more easily reviewable chunks.

[vyasr]

Closing since most of the changes are in other PRs.