remove `OhMyThreads.map` to avoid confusion

[aplavin]

I was struggling to get OhMyThreads to run actually in parallel, and the issue was in me typing OhMyThreads.map instead of OhMyThreads.tmap. Given that it's such an easy typo to make, and the code behaves the same aside from performance.

Can we avoid having OhMyThreads.map at all? Either shadow it, or use baremodule...