refactor(tvix/eval): use im::Vector for NixList representation

This is a persistent, structurally sharing data structure which is
more efficient in some of our use-cases. I have verified the
efficiency improvement using `hyperfine` repeatedly over expressions
on nixpkgs.

Lists are not the most performance-critical structure in Nix (that
would be attribute sets), but we can already see a small (~5-10%)
improvement.

Note that there are a handful of cases where we still go via `Vec`
that need to be fixed, most notable for `builtins.sort` which can not
currently be implemented directly using `im::Vector` because of a
restrictive type bound.

Change-Id: I237cc50cbd7629a046e5a5e4601fbb40355e551d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7670
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -772,7 +772,12 @@ mod pure_builtins {
 
     #[builtin("sort")]
     fn builtin_sort(vm: &mut VM, comparator: Value, list: Value) -> Result<Value, ErrorKind> {
-        let mut list = list.to_list()?.into_vec();
+        // TODO: the bound on the sort function in
+        // `im::Vector::sort_by` is `Fn(...)`, which means that we can
+        // not use the mutable VM inside of its closure, hence the
+        // dance via `Vec`. I think this is just an unnecessarily
+        // restrictive bound in `im`, not a functional requirement.
+        let mut list = list.to_list()?.into_iter().collect::<Vec<_>>();
 
         // Used to let errors "escape" from the sorting closure. If anything
         // ends up setting an error, it is returned from this function.