Multi-asset extensions to Sapling

[joebebel]

Hi all! Inspired by the ZIP for user defined assets, we have extended the demo a bit to be more complete, and may be found here

Since the circuit extensions are rather generic, hopefully they will be of broad interest.

Comments, criticism, feedback, and review are very welcome! (And I do expect some strong opinions ;) )

edit: Also, we are interested in potentially upstreaming some or all of the code, or otherwise contributing to the community