feat(tvix/eval): emit warnings with kinds and spans

In the past, we had `emit_warning` be no-op and we used `push_warnings` exclusively
but as we have consumers of this function, we need it to work somewhat.

Change-Id: I78a5ece199a473dec9ef5ea1fae60b36e35137b8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10477
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/vm/mod.rs

@@ -968,13 +968,11 @@ impl<'o> VM<'o> {
 
     /// Emit a warning with the given WarningKind and the source span
     /// of the current instruction.
-    pub fn emit_warning(&mut self, _kind: WarningKind) {
-        // TODO: put LightSpan in warning, calculate only *after* eval
-        // TODO: what to do with the spans?
-        // self.push_warning(EvalWarning {
-        //     kind,
-        //     span: self.current_span(),
-        // });
+    pub fn emit_warning(&mut self, kind: WarningKind) {
+        self.push_warning(EvalWarning {
+            kind,
+            span: self.get_span(),
+        });
     }
 
     /// Interpolate string fragments by popping the specified number of