This fixes issue #62.

The error message accidentally got removed in e0789039.

wp_apply often results in an of_val that should be simplified.

Unfortunately, we currently have to keep the unicode-space hack in some places because Coq still complains about the notation otherwise

This approach is originally by Robbert

There are certainly more places this is useful, but let's start with this simple test

Fix fixes issue #63.

Rename:

- prefix_of -> prefix and suffix_of -> suffix because that saves keystrokes
  in lemma names. However, keep the infix notations with l1 `prefix_of` l2 and
  l1 `suffix_of` l2 because those are easier to read.
- change the notation l1 `sublist` l2 into l1 `sublist_of` l2 to be consistent.
- rename contains -> submseteq and use the notation ⊆+

by Janno

Thanks to Janno.