Cleanup workload directory structure

[danleh]

Definitely not urgent and only for after the 3.0 release, but it's a bit of a mess right now: Some workloads don't have their own directory (e.g., tfjs-wasm is just inside the wasm/ directory); some Wasm workloads are under wasm/, some are not; and for those workloads in their own directory, the directory doesn't always match the workload name.

Let's:

• put all workloads under workloads/,
• put each workload in their own subdirectory, where the directory name matches precisely the workload name (except for those workloads that are closely related or have lots of files in common)
• don't put Wasm workloads under wasm/ but just next to the JS ones.