This means that the examples currently won't work, since the new
interface is not implemented for other functors besides powerset.

This exists in parallel to the old registeredFunctors list right now,
but will replace it once it is fully used.

This is an existential that can wrap any functor that implements the
refinement interface. It will be used to do proper desorting in the
future, but right now it's a big hack to support the old
infrastructure while slowly migrating to new-style functors.

Since the tests for `printSorts` were accidentally disabled, it had
several bugs:

  - No newlines between table rows
  - Non-friendly output of the `Sort` data type
  - Crash when trying to print empty functor expression

This gets hairy pretty fast... In contrast, keeping the inner functor
type universal and using existentials only when needed allows to drop
in different existential type in different places (e.g tests).

doctest doesn't respect the `default-extensions` field from the cabal
file, so we previously had to put every extension at the start of each
and every file so that doctests would compile.

Using the `cabal-doctest` package, we can pass the right options to
`doctest` at build time and move extension declarations to the cabal
file.

This will allow more flexibility and a cleaner parsing implementation
down the line.

This means that "names" (e.g node labels for FixedProduct) are now
parsed with the `lexeme` combinator (which consumes trailing
whitespace).

This improves parser errors by a wide margin, but requires all functor
expressions to be distinguishable with one token look ahead. This is
currently the case, but we may need to reconsider this issue.

This should reduce the build time in CI drastically.

Switches from X^G to G^X. The former would have been the syntax for a
mapping from the monoid, not into the monoid...

Unfortunately, moving the GHC-extension declaration to the cabal file
broke our doctests...

We now try to parse each functor and backtrack if necessary. This
allows individual functors to begin with the same symbol.

I use some extensions so often that it's tedious to put them into each
individual file.

This switches the parser for functor expressions (not yet morphisms)
from a YAML based one to a custom one, making those expressions much
more readable and giving individual functors greater flexibility to
define their own syntax.

processQueue was missing a 'while' loop around everything. With this,
the algorithm computes the correct result for example/example.yaml.

Yeah... leaving FIXMEs in the code and wondering why it doesn't
work.

The refinement algorithm is now complete, but (of course) sill buggy.