support arbitrary-length tuples as computed artifact input

[brendankenny]

follow up to #2705 and #2275

Computed artifacts can now take a list of artifacts of any length as input, but not of multiple different lengths for any one computed artifact. Supporting "here's a list of artifacts, cache on these" (regardless of length) would allow e.g. optional arguments to computed artifacts, and removal of the current requiredNumberOfArtifacts() requirement on any derived class that wants to support more than one input artifact.