Remove access to old size macros

[ncik-roberts]

Stop allowing use of the *size* family of macros. User code should instead use Allocated_*size*. This helps us be more confident that users will be notified if importing code that interprets a block's allocated size as its scannable size.

There are two exceptions:

• User code can explicitly request the old naming of macros by defining a magic macro. This makes some user code much easier to update. (It's encouraged to use the new names, as this will also force people to think about new code they import in the future.)
• The compiler distribution itself uses the *size* family of macros. This avoids some renaming churn in the compiler which could introduce the possibility of merge conflicts. A counterargument is: well, we should be confident that the compiler code has been vetted in the same way. (My preference is to rely on testing here.) I'm open to rethinking this, but my starting point is that it's better to avoid the merge conflicts.

Once #2653 is merged and user code is updated, we can merge this.