Initial conversion to Rust library

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.

src/error.rs

@@ -0,0 +1,45 @@
+use nom::error::{ErrorKind, ParseError as NomParseError};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum ParseError {
+    #[error("invalid ZZT file magic")]
+    InvalidMagic,
+
+    #[error("invalid tile count: {0}")]
+    InvalidTileCount(usize),
+
+    #[error("negative stat count")]
+    NegativeStatCount,
+
+    #[error("string too long")]
+    StringTooLong { max: u8 },
+
+    #[error("cannot encode character: {0}")]
+    EncodingError(char),
+
+    #[error("board data too large")]
+    BoardTooLarge,
+
+    #[error("nom error: {0}")]
+    NomError(String),
+}
+
+impl<I> NomParseError<I> for ParseError {
+    fn from_error_kind(_input: I, kind: ErrorKind) -> Self {
+        Self::NomError(kind.description().to_string())
+    }
+
+    fn append(_input: I, kind: ErrorKind, other: Self) -> Self {
+        Self::NomError(format!("{}: {:?}", other, kind))
+    }
+}
+
+impl From<nom::Err<ParseError>> for ParseError {
+    fn from(value: nom::Err<ParseError>) -> Self {
+        match value {
+            nom::Err::Error(e) | nom::Err::Failure(e) => e,
+            nom::Err::Incomplete(needed) => Self::NomError(format!("incomplete: {:?}", needed)),
+        }
+    }
+}