CT sizeof(+friends) for {.importc, completeStruct.} types, enable ABI static checks (#13926)

* -d:checkabi obsolete (ABI check now enforced); add `addTypeHeader` helper
* cleanups
* import sizeof at CT for {.completeType.}
* address comments; revert default enabling of -d:checkAbi for now
* mimportc_size_check.nim => msizeof5.nim; merge mabi_check.nim into msizeof5.nim; refactor
* all pragmas in errmsgs should be written: '.importc' (un-ambiguous and less verbose than {.importc.})

compiler/ast.nim

@@ -553,6 +553,9 @@ type
                            # If it has one, t.destructor is not nil.
     tfAcyclic # object type was annotated as .acyclic
     tfIncompleteStruct # treat this type as if it had sizeof(pointer)
+    tfCompleteStruct
+      # (for importc types); type is fully specified, allowing to compute
+      # sizeof, alignof, offsetof at CT
 
   TTypeFlags* = set[TTypeFlag]