Review `loop2` and other looping forms

[countvajhula]

loop2 seems odd. It appears to expect specifically two inputs, and produces specifically one output:

• the first input is a list of inputs
• the second input is the in-progress result
• the output is a single value

Should it instead support the inputs being any number of values? Should the output be any number of values? We probably can't do both here since there would be no way to differentiate inputs to be processed from in-progress outputs. Most likely, the output should be a single value (including a list), while the input should be any number of values (and the in-progress result can be fed forward as the first input).

More generally, when are loop and loop2 useful? Most existing usecases appear to be covered by amp and the folds, << and >>. Finally, feedback seems to cover remaining cases. When do we need loop and loop2? Is it perhaps that these could be considered lower-level forms with which to implement amp and >> / <<?