Make sure lists are printed with the proper separators as given by the context.

The `Context()->lists` object includes values for the separator to use for each list type.  These were not being honored for TeX output (and some string output). You can check the patch by using

```
Context()->lists->set(List=>{separator=>';'});

$L = List(1,2,3);
$F = Formula("x,1");

BEGIN_TEXT
\(\{$L->TeX\}\) and \(\{$F->TeX\}\)
END_TEXT
```

Without the patch, these should produce comma-separated lists; with the patch, they should be separated by semicolons.
This pull requests test correctly. I'm just wondering what this does, purely out of curiosity.