feat(tvix/eval): add method for emitting runtime warnings

This lets the VM emit warnings when it encounters situations that
should only be warned about at runtime.

For starters, this is used to pass through compilation warnings that
come up when `import` is used.

Change-Id: I0c4bc8c534d699999887c430d93629fadfa662c4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6868
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

corp/tvixbolt/src/main.rs

@@ -292,7 +292,18 @@ fn eval(trace: bool, code: &str) -> Output {
     };
 
     match result {
-        Ok(value) => writeln!(&mut out.output, "{}", value).unwrap(),
+        Ok(result) => {
+            for warning in result.warnings {
+                writeln!(
+                    &mut out.warnings,
+                    "{}\n",
+                    warning.fancy_format_str(&source).trim(),
+                )
+                .unwrap();
+            }
+
+            writeln!(&mut out.output, "{}", result.value).unwrap()
+        }
         Err(err) => writeln!(
             &mut out.runtime_errors,
             "{}",