build: update all crates to Rust 2024 edition

Largest change is in the FFI crate.
With 2024 (but not 2021) edition unsafe code
inside unsafe functions should be marked separately
so we can mark exactly the code that is unsafe.
Some CFFI functions even have no unsafe code inside.

Most interesting change is that .strdup()
functions are not marked as unsafe anymore.
They are allocating memory and return raw pointers,
but there is nothing unsafe about it.
Only using the returned raw pointers is unsafe.
This way calls to .strdup() don't have to be marked
with unsafe{} blocks.

format-flowed/Cargo.toml

@@ -2,7 +2,7 @@
 name = "format-flowed"
 version = "1.0.0"
 description = "format=flowed support"
-edition = "2021"
+edition = "2024"
 license = "MPL-2.0"
 
 keywords = ["email"]

format-flowed/src/lib.rs

@@ -78,11 +78,11 @@ pub fn format_flowed(text: &str) -> String {
 
         let mut prefix = prefix.to_string();
 
-        if quote_depth > 0 {
-            if let Some(s) = line.strip_prefix(' ') {
-                line = s;
-                prefix += " ";
-            }
+        if quote_depth > 0
+            && let Some(s) = line.strip_prefix(' ')
+        {
+            line = s;
+            prefix += " ";
         }
 
         result += &format_line_flowed(line, &prefix);
@@ -224,8 +224,7 @@ mod tests {
     fn test_unformat_flowed() {
         let text = "this is a very long message that should be wrapped using format=flowed and \n\
             unwrapped on the receiver";
-        let expected =
-            "this is a very long message that should be wrapped using format=flowed and \
+        let expected = "this is a very long message that should be wrapped using format=flowed and \
                         unwrapped on the receiver";
         assert_eq!(unformat_flowed(text, false), expected);
 
@@ -255,8 +254,7 @@ mod tests {
         assert_eq!(format_flowed_quote(quote), expected);
 
         let quote = "this is a very long quote that should be wrapped using format=flowed and unwrapped on the receiver";
-        let expected =
-            "> this is a very long quote that should be wrapped using format=flowed and \r\n\
+        let expected = "> this is a very long quote that should be wrapped using format=flowed and \r\n\
             > unwrapped on the receiver";
         assert_eq!(format_flowed_quote(quote), expected);
     }