refactor(tvix/eval): store spans instead of nodes in Warning/Error

Another step towards being able to report accurate errors. The codemap
spans contain strictly more accessible information, as they now retain
information about which input file something came from.

This required some shuffling around in the compiler to thread all the
right information to the right places.

Change-Id: I18ccfb20f07b0c33e1c4f51ca00cd09f7b2d19c6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6404
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/warnings.rs

@@ -11,6 +11,6 @@ pub enum WarningKind {
 
 #[derive(Debug)]
 pub struct EvalWarning {
-    pub node: rnix::SyntaxNode,
     pub kind: WarningKind,
+    pub span: codemap::Span,
 }