Add an ADSR envelope example to [slop~] and improve 'slew' example.

[porres]

I have been teaching and using [slop~] to implement an ADSR envelope since this object came out. I'd like to also add this as an example to [slop~] documentation.

Here's it as an abstraction, I would adapt it not to be an abstraction.

As for an abstraction, I should probably open a new issue, but I'd also like to propose yet another [adsr~] abstraction for the control examples that uses this mechanism.

[porres]

For the record, this is what an actual usual non linear envelope generator works, right? What do you people say?

[porres]

I'm also adding another patch for the slew limiting example to make it clear how it works

[porres]

And here's the envelope example