add FuncUMapDef

an UMapDef that doesn't create a synth but instead fires a function at
U:prepare and also at any parameter change
add the first FuncUMapDef: random_value
enable multichannel UMaps and allow more specs
add two multichannel UMaps: center_range and lo_hi

These umaps can be used on RangeSpec args. They transform lo and hi, or
center and range, to individual controls, each of which can get a
single-channel UMap.