Detect built-in procedures

[eoyoa]

For example, the + in (reduce + (list ...)) should act as a first-class function, not just as a value.

It should expand with the correct number of arguments.

Probably requires deeper integration with Scamper.

[eoyoa]

According to PM, it requires a lot of code changes to Scamper to fix.

For now, just do a quick fix and detect if the constant block that's about to be generated is a symbol, and just treat it as a special type of value.