saugns v0.3.7c. Rearrange code. Fix hang with -r 1.

Reduce design difference with 2020-06 "mgensys" a bit further;
split "generator" pre-allocation code into prealloc (like a
simpler runalloc), and merge voicegraph into prealloc. Also
rename generator to interp.

Fix hang with the useless, yet allowed, sample rate of 1 Hz
due to 0 samples being generated per "run" call, and time thus
never advancing for the interpreter. Simply end when 0 samples
are filled in the player code.
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