tclx: new package (tcl extension)

With the current Tcl binaries there is a crash on exit when running for i686. This crash can be replicated with:

```
$ winpty tclsh
% package require Tclx
8.4
% exit
```

Building with `-static-libgcc` fixes the crash. But this problem is supposed to be fixed on the not-yet-released Tcl binaries based on very recent mingw-runtime.