Support sizeof and alignof inside macros for stable types

[HertzDevil]

This PR enables the use of sizeof and alignof in macro expressions when the type's size and alignment are known to be stable, i.e. their values cannot be changed by adding new code. The idea was raised in this forum thread and possibly in other GitHub issues as well. With this PR you can write compile-time checks like:

{% unless sizeof(Void*) == sizeof(LibC::SizeT) %}
  {% raise "we don't allow the x86 Real Mode here" %}
{% end %}

All types are stable, except for the following:

• Non-primitive structs, e.g. Bytes
• ReferenceStorage instances
• Modules, e.g. GC (not to be confused with GC.class, all metaclasses are stable)
• Uninstantiated generic types, e.g. Array
• Annotations, e.g. x = uninitialized Flags (these "types" are ill-behaved anyway)
• StaticArray, Tuple, NamedTuple, and Union instances containing any unstable constituent types

typeof is disallowed, because we obviously cannot do any kind of non-macro semantic analysis inside macros.

Although the non-macro sizeof and alignof support constants, their macro counterparts do not, even when constant evaluates to a simple compile-time literal (A = 1; {{ sizeof(A) }}).