Use InstructionPtr instead of a program counter (usize)

[Robbepop]

This is a follow-up to #473.

An advantageous design is to change branch instructions to offset the program counter by a delta relative to the current program counter. This way we can avoid having the base pointer and only need to carry around a single pointer to the next instruction respectively per FuncFrame in the call stack.
Another implication with this proposed design is that returning from a function will no longer require to query the instructions of a FuncFrame via the CodeMap but only calling a function will have to do this, therefore call intense workloads will get faster due to having to do less in between function executions after returns.
Dropping the Instructions abstraction in the Executor will spare us 16 bytes there which probably improves cache friendliness of the Executor itself.

• Implement InstructionPtr abstraction and CodeMap API to access those.
• Embed new InstructionPtr into FuncFrame and replace pc: usize by it.
• Make Executor properly use InstructionPtr and properly load from CodeMap only upon calls but no longer on returns.
• Remove the Instructions type since it is of no use anymore.