autofix: Remove constructor from link_provider

A link_provider can accumulate link-free policies, but only via
sequential composition. That's because a link_provider represents a
policy that looks like this:

   (lfp ; r)

where lfp is a link-free policy, and r is a policy that's almost a
regular expression, when lfp is distributed across it. Associativity
matters here, so you can only extend lfp via sequential composition.

Related to #152, #155, and #158.
autofix: Revert change to Seq(NL, NL) case

This reverts the essential part of commit 7d240da0270, introduced in
pull request #155, which was intended to be a fix to #152. While it
fixed some of the errant behavior, it broke some tests as well.

Still in the process of figuring this out.
autofix: Get examples running again
autofix: Better-working regex_of_policy

This commit changes the way that regex_of_policy works, and fixes some
bugs along the way. regex_of_policy now does two passes. First, it
draws boxes around link-free policies, and link-free policy, link pairs.
Then the algorithm works as it did before. There's one exception to
that, which is the Seq(NL, NL) case now works as it should.

The dump and run commands for automaton also now guards according to the
switch the policy should run on.
Merge pull request #166 from frenetic-lang/autofix

Fixing Automaton