Allow TapedFunction as a special Instruction

[yebai]

We currently treat all instructions on a tape as primitive. There are use-cases (see here) where we need to selectively trace into instructions. One neat way of achieving that is to have a special instruction type that allows us to plug a tape A into another tape B, where tape A is considered as a standard instruction on tape B. This way, we can create a dedicated tape for an instruction that we want to trace into, and use it on another (parent) tape.

[KDr2]

Fixed by #100.

[KDr2]

Do we still need this feature?

[yebai]

We might still want to selectly convert some function calls into TapedFunctions. Let's revisit this later.

[yebai]

Probably not necessary after #138