Refactor a bunch of stuff around to allow for polymorphically surfacing
an EntityChar for all entities, and use this to write a generic
`entityMenu` function, which generates a menu from the chars of a list
of entities - and use that to fully implement (removing `undefined`)
menus for both attacking and picking things up when there are multiple
entities on the relevant tile.
Implement ToJSON and FromJSON for all of the various pieces of the game
state, and add a pair of functions saveGame/loadGame implementing a
prism to save the game as zlib-compressed JSON. To test this, there's
now Arbitrary, CoArbitrary, and Function instances for all the parts of
the game state - to get around circular imports with the concrete
entities this unfortunately is happening via orphan instances, plus an
hs-boot file to break a circular import that was just a little too hard
to remove by moving things around. Ugh.
When tracking message history, save messages associated with the turn
they were displayed on, which allows us to have the notion of the
"current turn's" messages (provided via a MonoComonad instance).
Add the beginnings of a generic prompt system, with exclusive support
atm for string prompts, and test it out by asking the character for
their name at startup
Add support for converting generated levels to walls, and merge one into
the entity map at the beginning of the game.
There's nothing here that guarantees the character ends up *inside* the
level though (they almost always don't) so that'll have to be slotted
into the level generation process.
Add a "previous message" command, triggered via ctrl+p.
I attempted here to get the message area to still take up a row of space
post-hiding the message, but failed - should revisit that at some point
