build: intergrate fuzz crate into workspace

This makes `fuzz` use the same lockfile
as the rest of the crates
and makes sure it fuzzes the same versions
of dependencies, e.g. `mailparse`.

src/lib.rs

@@ -116,6 +116,3 @@ pub const DCC_MIME_DEBUG: &str = "DCC_MIME_DEBUG";
 mod test_utils;
 #[cfg(test)]
 mod tests;
-
-#[cfg(fuzzing)]
-pub mod fuzzing;