Kill to_cstring with fire

I swear I already did this, see #273.

src/context.rs

@@ -310,12 +310,12 @@ pub unsafe fn dc_open(context: &Context, dbfile: &str, blobdir: Option<&str>) ->
     if 0 != dc_is_open(context) {
         return false;
     }
-    *context.dbfile.write().unwrap() = to_cstring(dbfile);
+    *context.dbfile.write().unwrap() = dbfile.strdup();
     if blobdir.is_some() && blobdir.unwrap().len() > 0 {
-        let dir = to_cstring(dc_ensure_no_slash_safe(blobdir.unwrap()));
+        let dir = dc_ensure_no_slash_safe(blobdir.unwrap()).strdup();
         *context.blobdir.write().unwrap() = dir;
     } else {
-        let dir = to_cstring(dbfile.to_string() + "-blobs");
+        let dir = (dbfile.to_string() + "-blobs").strdup();
         dc_create_folder(context, dir);
         *context.blobdir.write().unwrap() = dir;
     }

src/dc_securejoin.rs

@@ -45,12 +45,12 @@ pub unsafe fn dc_get_securejoin_qr(
     dc_ensure_secret_key_exists(context);
     invitenumber = dc_token_lookup(context, DC_TOKEN_INVITENUMBER, group_chat_id);
     if invitenumber.is_null() {
-        invitenumber = to_cstring(dc_create_id());
+        invitenumber = dc_create_id().strdup();
         dc_token_save(context, DC_TOKEN_INVITENUMBER, group_chat_id, invitenumber);
     }
     auth = dc_token_lookup(context, DC_TOKEN_AUTH, group_chat_id);
     if auth.is_null() {
-        auth = to_cstring(dc_create_id());
+        auth = dc_create_id().strdup();
         dc_token_save(context, DC_TOKEN_AUTH, group_chat_id, auth);
     }
     let self_addr = context.sql.get_config(context, "configured_addr");

src/dc_tools.rs

@@ -775,7 +775,7 @@ pub unsafe fn dc_create_outgoing_rfc724_mid(
     - the message ID should be globally unique
     - do not add a counter or any private data as as this may give unneeded information to the receiver	*/
     let mut rand1: *mut libc::c_char = 0 as *mut libc::c_char;
-    let rand2: *mut libc::c_char = to_cstring(dc_create_id());
+    let rand2: *mut libc::c_char = dc_create_id().strdup();
     let ret: *mut libc::c_char;
     let mut at_hostname: *const libc::c_char = strchr(from_addr, '@' as i32);
     if at_hostname.is_null() {
@@ -789,7 +789,7 @@ pub unsafe fn dc_create_outgoing_rfc724_mid(
             at_hostname,
         )
     } else {
-        rand1 = to_cstring(dc_create_id());
+        rand1 = dc_create_id().strdup();
         ret = dc_mprintf(
             b"Mr.%s.%s%s\x00" as *const u8 as *const libc::c_char,
             rand1,
@@ -1545,12 +1545,6 @@ impl<T: AsRef<str>> StrExt for T {
     }
 }
 
-/// Needs to free the result after use!
-pub unsafe fn to_cstring<S: AsRef<str>>(s: S) -> *mut libc::c_char {
-    let cstr = CString::new(s.as_ref()).expect("invalid string converted");
-    dc_strdup(cstr.as_ref().as_ptr())
-}
-
 pub fn to_string(s: *const libc::c_char) -> String {
     if s.is_null() {
         return "".into();