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.})

tests/misc/tsizeof2.nim

@@ -1,5 +1,5 @@
 discard """
-errormsg: "cannot evaluate 'sizeof' because its type is not defined completely"
+errormsg: "'sizeof' requires '.importc' types to be '.completeStruct'"
 line: 9
 """