Signal interfaces extensive allocations

[dudk]

The following methods return an interface values and cause heap allocations which result in additional load on GC:

• Append
• AppendSample
• Slice

There is no real solution to this in current Go. It just should be clear that users can't expect performance of raw Go slices with this methods.

[dudk]

Memory profile of mixer with 100 lines that was using AppendSample function.

[dudk]

Thу solution to extensive allocations is to avoid the return of interfaces where possible. Only Slice function should still return result because it's aligned with semantics - new signal buffer is returned.

[dudk]

This was fixed in #32