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`.

fuzz/fuzz_targets/fuzz_format_flowed.rs

@@ -9,7 +9,7 @@ fn round_trip(input: &str) -> String {
 
 fn main() {
     check!().for_each(|data: &[u8]| {
-        if let Ok(input) = std::str::from_utf8(data.into()) {
+        if let Ok(input) = std::str::from_utf8(data) {
             let input = input.trim().to_string();
 
             // Only consider inputs that are the result of unformatting format=flowed text.

fuzz/fuzz_targets/fuzz_simplify.rs

@@ -1,13 +0,0 @@
-use bolero::check;
-
-use deltachat::fuzzing::simplify;
-
-fn main() {
-    check!().for_each(|data: &[u8]| match String::from_utf8(data.to_vec()) {
-        Ok(input) => {
-            simplify(input.clone(), true);
-            simplify(input, false);
-        }
-        Err(_err) => {}
-    });
-}