refactor(tvix/eval): persistent, memory-sharing OrdMap for NixAttrs

This uses the `im::OrdMap` for `NixAttrs` to enable sharing of memory
between different iterations of a map.

This slightly speeds up eval, but not significantly. Future work might
include benchmarking whether using a `HashMap` and only ordering in
cases where order is actually required would help.

This switches to a fork of `im` that fixes some bugs with its OrdMap
implementation.

Change-Id: I2f6a5ff471b6d508c1e8a98b13f889f49c0d9537
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7676
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -601,7 +601,7 @@ fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use im::vector;
+    use imbl::vector;
 
     mod nix_eq {
         use crate::observer::NoOpObserver;