Benchmarks: document their purpose.

Most benchmarks are defined to produce a cost function. However, some of them are not used to this end, and their purpose is not properly documented.

We can formally add the documentation and show it in the display info for benchmark Snoop command.

• Set the generated code destination for all benchmarks that have a cost function in the protocol.
• Define a lib_benchmark.Benchmark.purpose = Generate_code of string | Other_purpose of string type.
• Change the lib_benchmark.Benchmark.generated_code_destination : string option field to lib_benchmark.Benchmark.purpose : purpose.
  • And adapt benchmark accordingly, setting purpose = Other_purpose "TODO" to those that had generated_code_destination = None.
  • Also adapt the display command.
• Correctly set the purpose of the benchmarks where purpose = Other_purpose "TODO".