Support synthesis queries referencing multiple outputs

[gussmith23]

Currently, Lakeroad makes assumptions about there being exactly one output of the spec and the sketch. It asserts those outputs are equal in rosette-synthesize. However, that's not necessary; there can be multiple outputs of the expression.

#502 gets us closer to supporting this by adding an unimplemented output-ports field to rosette-synthesize.

• Do not assume what output we use in sketches; e.g. DSP sketch should return all outputs in a map
• Use output signal flags to populate output-ports argument of rosette-synthesize
• Make multiple assertions in rosette-synthesize