File deduplication (#6332)

When receiving messages, blobs will be deduplicated with the new
function `create_and_deduplicate_from_bytes()`. For sending files, this
adds a new function `set_file_and_deduplicate()` instead of
deduplicating by default.

This is for
https://github.com/deltachat/deltachat-core-rust/issues/6265; read the
issue description there for more details.

TODO:
- [x] Set files as read-only
- [x] Don't do a write when the file is already identical
- [x] The first 32 chars or so of the 64-character hash are enough. I
calculated that if 10b people (i.e. all of humanity) use DC, and each of
them has 200k distinct blob files (I have 4k in my day-to-day account),
and we used 20 chars, then the expected value for the number of name
collisions would be ~0.0002 (and the probability that there is a least
one name collision is lower than that) [^1]. I added 12 more characters
to be on the super safe side, but this wouldn't be necessary and I could
also make it 20 instead of 32.
- Not 100% sure whether that's necessary at all - it would mainly be
necessary if we might hit a length limit on some file systems (the
blobdir is usually sth like
`accounts/2ff9fc096d2f46b6832b24a1ed99c0d6/dc.db-blobs` (53 chars), plus
64 chars for the filename would be 117).
- [x] "touch" the files to prevent them from being deleted
- [x] TODOs in the code

For later PRs:
- Replace `BlobObject::create(…)` with
`BlobObject::create_and_deduplicate(…)` in order to deduplicate
everytime core creates a file
- Modify JsonRPC to deduplicate blob files
- Possibly rename BlobObject.name to BlobObject.file in order to prevent
confusion (because `name` usually means "user-visible-name", not "name
of the file on disk").

[^1]: Calculated with both https://printfn.github.io/fend/ and
https://www.geogebra.org/calculator, both of which came to the same
result
([1](https://github.com/user-attachments/assets/bbb62550-3781-48b5-88b1-ba0e29c28c0d),

[2](https://github.com/user-attachments/assets/82171212-b797-4117-a39f-0e132eac7252))

---------

Co-authored-by: l <link2xt@testrun.org>

src/mimeparser/mimeparser_tests.rs

@@ -1373,8 +1373,8 @@ async fn test_x_microsoft_original_message_id() {
                 \n\
                 Does it work with outlook now?\n\
                 ", None)
-        .await
-        .unwrap();
+            .await
+            .unwrap();
     assert_eq!(
         message.get_rfc724_mid(),
         Some("Mr.6Dx7ITn4w38.n9j7epIcuQI@outlook.com".to_string())
@@ -1505,8 +1505,8 @@ async fn test_ignore_read_receipt_to_self() -> Result<()> {
     // Due to a bug in the old version running on the other device, Alice receives a read
     // receipt from self.
     receive_imf(
-        &alice,
-            "Received: (Postfix, from userid 1000); Mon, 4 Dec 2006 14:51:39 +0100 (CET)\n\
+            &alice,
+                "Received: (Postfix, from userid 1000); Mon, 4 Dec 2006 14:51:39 +0100 (CET)\n\
                  From: alice@example.org\n\
                  To: alice@example.org\n\
                  Subject: message opened\n\
@@ -1532,10 +1532,10 @@ async fn test_ignore_read_receipt_to_self() -> Result<()> {
                  \n\
                  \n\
                  --SNIPP--"
-        .as_bytes(),
-        false,
-    )
-    .await?;
+            .as_bytes(),
+            false,
+        )
+        .await?;
 
     // Check that the state has not changed to `MessageState::OutMdnRcvd`.
     let msg = Message::load_from_db(&alice, msg.id).await?;
@@ -1601,8 +1601,8 @@ async fn test_receive_eml() -> Result<()> {
         "this is a classic email – I attached the .EML file".to_string()
     );
     assert_eq!(
-        mime_message.parts[0].param.get(Param::File),
-        Some("$BLOBDIR/.eml")
+        mime_message.parts[0].param.get(Param::Filename),
+        Some(".eml")
     );
 
     assert_eq!(mime_message.parts[0].org_filename, Some(".eml".to_string()));