Add exponential control function to synthio.Envelope

[d1ffeq-diy]

At the moment, synthio.Envelope creates Attack and Release slopes in a linear fashion, however due to logarithmic human perception of loudness, this doesn't sound as natural or musical as "logarithmic" (non-linear) envelopes do. That's why ADSR+VCA parts of synthesizers don't use linear response. Another example is Volume potentiometers in audio, typically logarithmic for better control.

What could work best in this case is 'exponential' flag for Envelope parameters, which is False by default and can be set to True for approximately log response.

Unfortunately, Note only accepts Envelope object as envelope parameter so there's no way to solve this currently.