Fix erratic X86FunctionParser behavior

- Only relevant in 32-bit mode
- Don't read RET operand value unless there is an operand
Revise API to provide both function and variable exports

Also move arguments into GumFooDetails structs across the board for
consistency and to allow future additions without breaking the API or
ABI.
Update MemoryMap to the new process API
Update Script to the new process API
Update vapi to the new API and fill some of the existing blanks
Update gum-heap library to the new process API
Update Darwin backend to the new process API