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/ccgmerge.nim

@@ -48,6 +48,10 @@ const
   NimMergeEndMark = "/*\tNIM_merge_END:*/"
 
 proc genSectionStart*(fs: TCFileSection; conf: ConfigRef): Rope =
+  # useful for debugging and only adds at most a few lines in each file
+  result.add("\n/* section: ")
+  result.add(CFileSectionNames[fs])
+  result.add(" */\n")
   if compilationCachePresent(conf):
     result = nil
     result.add("\n/*\t")