Buffer: Add support for fill() with pre-determined waveforms

[ideoforms]

Signature: void Buffer::fill(std::string waveform, int frequency = 0)

e.g.: buf.fill("sine", 440) fills the buffer with a 440Hz sine wave.

Waveforms should include at least sine, triangle, saw, square, whitenoise, pinknoise.

If frequency is zero, fills the buffer with a single cycle of the waveform.