feat(tvix/eval): Initial resolution of <...> paths

This commit implements (lazy) resolution of `<...>` paths via either the
NIX_PATH environment variable, or the -I command-line flag - both
handled via EvalOptions. As a result, EvalOptions can no longer derive
Copy, meaning we have to clone it at each line of the repl - this is
probably not a huge deal as repl performance is not exactly an inner
loop and we're not cloning very much.

Internally, this works by creating a thunk which pushes a constant
containing the string inside the brackets to the stack, then a new
opcode to resolve that path via the `NixPath`. To get that opcode to
work, we now have to pass in the NixPath when constructing the VM.

This (intentionally) leaves out proper implementation of path resolution
via `findFile` (cppnix just calls whatever identifier called findFile is
in scope!!!) as that's widely considered a bit of a misfeature, but if
we do decide to implement that down the road it likely wouldn't be more
than a few extra ops within the thunk introduced here.

Change-Id: Ibc979b7e425b65cbe88599940520239a4a10cee2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6918
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -410,7 +410,7 @@ mod tests {
         #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
         fn reflexive(x: Value) {
             let mut observer = NoOpObserver {};
-            let mut vm = VM::new(&mut observer);
+            let mut vm = VM::new(Default::default(), &mut observer);
 
             assert!(x.nix_eq(&x, &mut vm).unwrap())
         }
@@ -418,7 +418,7 @@ mod tests {
         #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
         fn symmetric(x: Value, y: Value) {
             let mut observer = NoOpObserver {};
-            let mut vm = VM::new(&mut observer);
+            let mut vm = VM::new(Default::default(), &mut observer);
 
             assert_eq!(
                 x.nix_eq(&y, &mut vm).unwrap(),
@@ -429,7 +429,7 @@ mod tests {
         #[proptest(ProptestConfig { cases: 5, ..Default::default() })]
         fn transitive(x: Value, y: Value, z: Value) {
             let mut observer = NoOpObserver {};
-            let mut vm = VM::new(&mut observer);
+            let mut vm = VM::new(Default::default(), &mut observer);
 
             if x.nix_eq(&y, &mut vm).unwrap() && y.nix_eq(&z, &mut vm).unwrap() {
                 assert!(x.nix_eq(&z, &mut vm).unwrap())
@@ -439,7 +439,7 @@ mod tests {
         #[test]
         fn list_int_float_fungibility() {
             let mut observer = NoOpObserver {};
-            let mut vm = VM::new(&mut observer);
+            let mut vm = VM::new(Default::default(), &mut observer);
 
             let v1 = Value::List(NixList::from(vec![Value::Integer(1)]));
             let v2 = Value::List(NixList::from(vec![Value::Float(1.0)]));