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
Implement messages almost the same as in the Rust version, only with
YAML instead of TOML this time, and a regular old mustache template
instead of something handrolled. Besides that, pretty much everything
here is the same.
