parameter tree

[wrl]

currently, parameters are assembled in a flat array, which is how VST2 handles them. however, VST3 and AU support hierarchical arrangements of parameters addressed by opaque u32 IDs, which keeps automation stable when a plugin adds or rearranges parameters.

the model! procmacro has sufficient information to assemble parameters into a tree and should do so. there will also need to be a facility for manually specifying a linear order of parameters to support stable inter-version automation for VST2 (maybe LV2) plugins.