Performance of field calculus constructs

[metaphori]

• The implementation of branch on top of aggregate is nice by a logical point of view; however, aggregate is currently implemented by looking at the stack trace, which is an expensive operation.
  For this reason, it makes sense to implement branch directly in the interpreter as a core construct rather than as a derived one.
• FoldhoodPlus is implemented on top of foldhood: this means that the given expression is executed also for the current device, though its result is discarded in favor of the init value of the fold.