This is purely based on a set difference, i.e. it won't catch stupid
things like a map linking to itself, a map link going only one-way, etc.

Also, it only handles map links; it doesn't check if all ressource files
referenced by a map actually exist.

also, configurable log level, which only required relaxing the type
system once!

(+ checking that paths don't run outside of respositories)

This now checks if relative paths are still inside the repository, as a
general safety mechanism to stop the linter from accidentally reading
other things, as well as a nice hint for users.

it is no longer an Either since that wasn't used anyways, but is now
also a Reader.

apparently i couldn't read or something?

it's almost as if there's some structure to this code!

There's now a Lint type, which may be either a "true lint" (which is a
Hint, which contains some message and level of severity), or a Depends,
which indicates that this map depends on some ressource or other (and is
otherwise treated as a special info Hint in all other cases)

/finally/ figured out that all properties just look like {name, value,
type} so now that's abstracted away and Properties.hs doesn't look like
javascript anymore

Adds a PrettyPrint typeclass which operates on Text and should replace
Show, since constantly converting strings from linked lists to arrays
seems somewhat silly.

This makes map loading (and parsing) part of the linter, and also makes
it return "general lints" and nothing else in case that failed.

Possibly a sum type would be nicer here, but I guess it's not really
important since everything ends up as json anyways?

input options are mostly dummies for now, but some work (e.g. --inpath
and --json). Lints can now be optionally printed as json to be
reasonably machine-readable (and the json can be pretty-printed to make
it human-readable again …).

(also renaming things now that concepts seem a bit clearer)

Lots of monads everywhere, hurray!

(unfortunately, workadventure maps don't quite form a category; they lack composition …)

Credits:
 - the example.json file is by TabascoEye (with some modifications for testing purposes)
 - the Tiled module is forked from aeson-tiled on hackage, since that package didn't
   handle custom layer properties correctly