A lot of this is deleting zztext-specific functionality, such as string
mappings that go beyond official names for things. It's okay, zztext
will cope.
I'm spinning zztff off as its own Rust crate, separate from my zztext
project (which will soon consume it). This commit is based on running
filter-repo on the zztext project, followed by the minimal work to get
it to build separately.