It no longer requires the functions on both sides of the relation
to be syntactically the same.

This fixes issue #65.

This approach is originally by Robbert

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 ⊆+

This patch was created using

  find -name *.v | xargs -L 1 awk -i inplace '{from = 0} /^From/{ from = 1; ever_from = 1} { if (from == 0 && seen == 0 && ever_from == 1) { print "Set Default Proof Using \"Type*\"."; seen = 1 } }1 '

and some minor manual editing

Also make names more consistent.

We typically use the _1 and _2 suffix to denote individual directions
of a lemmas that is a biimplication.

case H; clear H would fail when H is dependent whereas destruct H
would succeed on that, but just not clear it.

The case tactic is faster than destruct.

Example:

Goal
  ¬False → ¬False → ¬False → ¬False → ¬False → ¬False → ¬False →
  False.
Proof.
  intros. done. (* takes very long *)

Using this new definition we can express being contractive using a
Proper. This has the following advantages:

- It makes it easier to state that a function with multiple arguments
  is contractive (in all or some arguments).
- A solve_contractive tactic can be implemented by extending the
  solve_proper tactic.