refactor(tvix/eval): remove use of imbl::OrdMap

Removes imbl::OrdMap in favour of an Rc over the standard library's BTreeMap,
which allows us to drop the imbl dependency completely.

In my local tests this is actually slightly faster for `hello` and `firefox`.

Change-Id: Ic9597ead4e98bf9530f290c6a94a3c5c3efd0acc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12201
Reviewed-by: aspen <root@gws.fyi>
Tested-by: BuildkiteCI

tvix/glue/benches/eval.rs

@@ -60,7 +60,7 @@ fn interpret(code: &str) {
     let eval = eval_builder.build();
     let result = eval.evaluate(code, None);
 
-    assert!(result.errors.is_empty());
+    assert!(result.errors.is_empty(), "{:#?}", result.errors);
 }
 
 fn eval_nixpkgs(c: &mut Criterion) {