saugns v0.5.6c. Ratio carrier selection for modulators. Fix.

Language changes:
 * Frequency. Ratio carrier selection for modulators. Every
   modulator now has the `r.c` option; it allows overriding
   which carrier's frequency is multiplied by the `r` value
   (by default going up one level, `r.c-1`). Changing it to
   0 or a positive value will go inward, from the root/top.
   Values out of range are clamped to the range, from root,
   to the level just above the modulator being set.

Fix v0.5.6 edge case bug for frequency parameter sweeps; if
using ratio for goal but not for state or vice-versa (mixed
rather than consistent selection), the newer handling could
mess up the signal. Scripts like the below are now fixed.

"W f[W f10 a440] t10 p[R f440 r[g1]]"