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/attrs/tests.rs

@@ -56,7 +56,7 @@ fn test_simple_attrs() {
     .expect("simple attr construction should succeed");
 
     assert!(
-        matches!(attrs, NixAttrs(AttrsRep::Map(_))),
+        matches!(attrs, NixAttrs(AttrsRep::Im(_))),
         "simple attribute set should use map representation",
     )
 }