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/sizealignoffsetimpl.nim

@@ -382,7 +382,8 @@ proc computeSizeAlign(conf: ConfigRef; typ: PType) =
         computeObjectOffsetsFoldFunction(conf, typ.n, false, accum)
       let paddingAtEnd = int16(accum.finish())
       if typ.sym != nil and
-         typ.sym.flags * {sfCompilerProc, sfImportc} == {sfImportc}:
+         typ.sym.flags * {sfCompilerProc, sfImportc} == {sfImportc} and
+         tfCompleteStruct notin typ.flags:
         typ.size = szUnknownSize
         typ.align = szUnknownSize
         typ.paddingAtEnd = szUnknownSize