This commit introduces nullable types `T?` that are
distinct from non-nullable `T`.
Example: `int?` (int or null) and `int` are different now.
Previously, `null` could be assigned to any primitive type.
Now, it can be assigned only to `T?`.
A non-null assertion operator `!` was also introduced,
similar to `!` in TypeScript and `!!` in Kotlin.
If `int?` still occupies 1 stack slot, `(int,int)?` and
other nullable tensors occupy N+1 slots, the last for
"null precedence". `v == null` actually compares that slot.
Assigning `(int,int)` to `(int,int)?` implicitly creates
a null presence slot. Assigning `null` to `(int,int)?` widens
this null value to 3 slots. This is called "type transitioning".
All stdlib functions prototypes have been updated to reflect
whether they return/accept a nullable or a strict value.
This commit also contains refactoring from `const FunctionData*`
to `FunctionPtr` and similar.
