feat(common): Implement RelationalLinkedChunk.

A `RelationalLinkedChunk` is like a `LinkedChunk` but with a relational
layout, similar to what we would have in a database.

This is used by memory stores. The idea is to have a data layout that
is similar for memory stores and for relational database stores, to
represent a `LinkedChunk`.

This type is also designed to receive `Update`. Applying `Update`s
directly on a `LinkedChunk` is not ideal and particularly not trivial
as the `Update`s do _not_ match the internal data layout of the
`LinkedChunk`, they have been designed for storages, like a relational
database for example.

This type is not as performant as `LinkedChunk` (in terms of memory
layout, CPU caches etc.). It is only designed to be used in memory
stores, which are mostly used for test purposes or light usages of the
SDK.

crates/matrix-sdk-common/src/linked_chunk/mod.rs

@@ -93,6 +93,7 @@ macro_rules! assert_items_eq {
 }
 
 mod as_vector;
+pub mod relational;
 mod updates;
 
 use std::{
@@ -933,7 +934,7 @@ impl ChunkIdentifierGenerator {
 /// Learn more with [`ChunkIdentifierGenerator`].
 #[derive(Copy, Clone, Debug, PartialEq)]
 #[repr(transparent)]
-pub struct ChunkIdentifier(u64);
+pub struct ChunkIdentifier(pub(super) u64);
 
 impl PartialEq<u64> for ChunkIdentifier {
     fn eq(&self, other: &u64) -> bool {
@@ -945,7 +946,7 @@ impl PartialEq<u64> for ChunkIdentifier {
 ///
 /// It's a pair of a chunk position and an item index.
 #[derive(Copy, Clone, Debug, PartialEq)]
-pub struct Position(ChunkIdentifier, usize);
+pub struct Position(pub(super) ChunkIdentifier, pub(super) usize);
 
 impl Position {
     /// Get the chunk identifier of the item.
@@ -966,6 +967,16 @@ impl Position {
     pub fn decrement_index(&mut self) {
         self.1 = self.1.checked_sub(1).expect("Cannot decrement the index because it's already 0");
     }
+
+    /// Increment the index part (see [`Self::index`]), i.e. add 1.
+    ///
+    /// # Panic
+    ///
+    /// This method will panic if it will overflow, i.e. if the index is larger
+    /// than `usize::MAX`.
+    pub fn increment_index(&mut self) {
+        self.1 = self.1.checked_add(1).expect("Cannot increment the index because it's too large");
+    }
 }
 
 /// An iterator over a [`LinkedChunk`] that traverses the chunk in backward