Compile first-class functions to classes

[NeilKleistGao]

• Create flag :defunc for defunctionalization
• Collect functions and generate corresponding function classes with the call function
• Substitute all first-class functions with corresponding class instantiation
• Invoke the call function for each first-class function

[LPTK]

Cool, but this is not defunctionalization, it's just compiling first class functions to classes. (That's fine, it's what we need, but the terminology is wrong and should be changed.)