Scale "freq" and "sustain" with speed in sclang

[ndr-brt]

This issues comes from a discussion in #241.
The necessity is to make sc synth events behaviour more similar to samples.

The proposal is to scale the freq by speed outside the synthdef like this:
freq = freq * speed;
And, to change also the synth sustain like:
sustain = sustain / speed.abs

This would allow to have speed scaling handled out of the boxy in a synthdef definition, but on the other hands would not be possible to continuously modify speed, and would be a breaking change.

[telephon]

Thanks! Let me add:

Independent of how we implement this (in each SynthDef or in superdirt itself, this may be e breaking change unless we make it switchable.