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/string.rs

@@ -3,6 +3,7 @@
 use smol_str::SmolStr;
 use std::hash::Hash;
 use std::ops::Deref;
+use std::path::Path;
 use std::{borrow::Cow, fmt::Display, str::Chars};
 
 #[derive(Clone, Debug)]
@@ -185,6 +186,12 @@ impl AsRef<str> for NixString {
     }
 }
 
+impl AsRef<Path> for NixString {
+    fn as_ref(&self) -> &Path {
+        self.as_str().as_ref()
+    }
+}
+
 impl Deref for NixString {
     type Target = str;