Add timing estimates as comments.
Add std::function example.
Enable advanced scheduling directives.

Added cases that match real usage patterns:

1. Defining and compiling the whole pipeline every time you want to run it (i.e. in the benchmarking loop)
2. Defining the pipeline outside the benchmarking loop, and realizing it repeatedly.
3. (optional) Same as 2), but calling compile_jit() outside the loop, saying what it does, and saying why the time isn't actually different to case 2 (benchmark() runs multiple times and takes a min, and realize only compiiles on the first run)
4. Compiling to a callable outside the benchmarking loop and showing that it has lower overhead than case 3 (if indeed it does. If not we may need to change the example so that it does, e.g. by adding a real input buffer.)