fix(tvix/eval): support string identifiers in inherits

This updates rnix-parser to a version where inherits provide an
iterator over `ast::Attr` instead of `ast::Ident`, which mirrors the
behaviour of Nix (inherits can have (statically known) strings as
their identifiers).

This actually required some fairly significant code reshuffling in the
compiler, as there was an implicit assumption in many places that we
would have an `ast::Ident` node available when dealing with variable
access (which is then explicitly only not true in this case).

Change-Id: I12f1e786c0030c85107b1aa409bd49adb5465546
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6747
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/scope.rs

@@ -105,7 +105,7 @@ pub enum UpvalueKind {
 #[derive(Clone, Debug)]
 pub struct Upvalue {
     pub kind: UpvalueKind,
-    pub node: rnix::ast::Ident,
+    pub span: codemap::Span,
 }
 
 /// Represents the index of a local in the scope's local array, which