Multiple aggregations

[bachdavi]

• Add record for multiple aggregations
• Aggregations now pass the argument symbols as a vector, also single aggregations as they are needed to reorder the key – value syms after the aggregation
• Projection now only gets a deduped version of the :find-syms. Prior led to overflow panics in the backends tuples_by_symbols(). Projection would try to project nested plans bindings onto symbols in the find clause which have a potential higher multiplicity due to several aggregations. So the nested bindings were smaller in number than Projections variables.
• Add fn_exp concat