Triple operands for if instruction encoding

[raviqqe]

Problem

if instruction encoding with skip instructions is not efficient and not identical to the original in-memory format.

Solution

Use triple operands. A continuation operand is appended to then and else branches on decoding.

• then
• else
• continuation

References

• This depends on exntension of "return" instructions.
  • `null` as return #88

[raviqqe]

Replaced by skip instruction encoding. #155