libs/sig: introduce a signal generator abstraction

This adds a small framework of sorts for creating and composing signal
generators.

Two generators are implemented at this time; sig_ops_sin and sig_ops_mult

sig_ops_sin accepts a hz variable and will produce a sine wave of that
frequency.

sig_ops_mult accepts two sig_t generators and multiplies their outputs

Callers may construct their own sig_ops_t ops structs and supply them to
sig_new(), but it's expected that libs/sig will grow a collection of
commonly used generators which can then be used by simply passing their
sig_ops_$foo to sig_new().

See the test code at the bottom of libs/sig/sig.c for some contrived
sample usage.  Note by composing multiple sig_ops_sin generators with
a sig_ops_mult generator, one can already easily construct a synth-like
LFO generator.

Some obvious todos are to add triangle/sawtooth/square wave generators.

More compositional generators may be interesting as well, like additive
and subtractive for example.  Those will need to implement clipping, as
it's expected that the generators *always* stay within unity (0-1).

No modules use this yet, but I expect to wire this up to rtv for driving
knobs.

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