Envelope detector

[olofson]

Envelope detector unit. The most basic implementation would track the peak level of one or more audio inputs, and present that as a readable register.

A more useful implementation would probably implement configurable filtering, and an optional ramped control output.

Technically, this unit should not have audio outputs, though it may be handy to have optional ones (requiring that they match the number of inputs, if used), especially until explicit wiring support is implemented.