Added 'fm1', 'fm2', 'fm3' and 'fm4' oscillator units

Added four simple FM (actually PM) oscillators with 1 through 4 operators
chained. The operators each have pitch, amplitude and feedback controls,
and there is a common 'phase' register that sets all operators at once.

The feedback controls allow operators to phase modulate themselves, which
results in saw-tooth-like waveforms.

The unit uses linear interpolation and 8x oversampling when compiled with
A2_HIFI.

The waveforms are created from a process-wide precalculated sine table
with 2048 entries, which is enough that the oscillator sounds quite OK
most of the time, even with interpolation and oversampling disabled.

Closes #155.