Implement a set of rules for transitioning values from compile-time to run-time

[zachwolfe]

Same-size

• All explicitly-sized integers should trivially transfer to run-time
• internal compiler data structures such as types and literals should not transfer, period
• structs and enums should move over trivially, as long as their components can all be moved over trivially
• ...

Pointer size

• a pointer value should transfer only in one of the following circumstances:
  • it was derived from a value returned by intern_static_data
  • it was derived from an integer literal (e.g., for memory-mapped IO)
• usize and isize should transfer as long as the values are representable in the target architecture's pointer-size int

[zachwolfe]

Replaced by #116