@jpcima already provided the curves in #96.

• Add the curve index and step to the CCModifier structure.
• Change the modifier lambdas to take these into account when necessary, with proper references to the curves.
• Boolean check (?) in the modifier loop to handle the quantized case.

If necessary, maybe it's time to revisit having a transparent helper for the modifier loop (see e.g. https://godbolt.org/z/7ocfDj to add static assertions on the lambda parameter). In particular, I guess stepped-envelopes should not be smoothed too?