Support different (input) arities in sources for the same table

[mmarx]

Currently, all sources for a table are required to match on input arity, since we reuse the TupleBuffer. This precludes, e.g., combining incremental imports with facts in the program.

[mmarx]

Probably easiest would be to just reconfigure the TupleBuffer for each source (we already need to set/clear the filters anyway).