Improve the `fn` pointer types

The current function pointer types have some downsides:

1. They assume that all functions are static code that lives forever. This isn't true for JITs or DLLs.
2. They are always non-nullable, safe pointers. This may not be what you want -- particularly the nullable aspect. This is connected to the previous point, because it implies that there is no place to attach a lifetime in the current syntax.

This is a "postponement issue". There have been various RFCs aiming to improve this:

• #252 -- a fairly straight-forward scheme to add lifetimes to fn types.
• #883 -- a simple scheme to change the syntax of fn to &'static fn for forwards compatibility.
• #996 -- a more involved scheme which introduced a kind of unsized type.

It should be possible to adapt these schemes backwards compatibly.

No source files, tests, or entry points are named. Start by reading RFCs #252, #883, and #996, then compare their lifetime, nullability, and syntax proposals to determine whether one can be adapted backwards compatibly; done requires a concrete, agreed direction rather than just postponement.