Fix marking nodes as dirty when inserting into an existing hashmap entry

[ealmloff]

Inserting into a hashmap or btreemap currently only marks the hashmap itself as dirty which will cause any get or iter operations to rerun, but it doesn't mark the existing value at the entry as dirty. This PR fixes that issue.

Fixes #4956

[jkelleyrtp]

Re: the comment on the changed bit, we do have the Ord bound which implies Eq + PartialEq - is there no way to use those to determine if it can be marked dirty?

[ealmloff]

Re: the comment on the changed bit, we do have the Ord bound which implies Eq + PartialEq - is there no way to use those to determine if it can be marked dirty?

Not that I can think of. Store selectors need to be stable over time and copy. Currently they are [u32; LEN] where each entry is either a list index or a hash of the type. The other index I could think of for BTrees is the order in the tree, but that is not stable over time if you insert an element before the element you are holding. For example, that would break this case:

let tree = store_hashmap();
// Creates path [0]
tree.insert(1, String::new());
// Holds path [0] in the selector subscription tree
let first = tree.get(1);
// Creates path [0] and shift the original value to the second item
tree.insert(0, "wrong value".to_string());
// This now references (the subscriptions for) "wrong value"
first.read();