feat(tvix/eval): add mechanism for emitting warnings from compiler

These can be used predominantly to emit warnings about things that the
compiler can infer, such as deprecated language features.

Change-Id: I3649c625459d7f3f95cdf42d5c651d23d66569ec
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6174
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

tvix/eval/src/warnings.rs

@@ -0,0 +1,11 @@
+/// Warnings are emitted in cases where code passed to Tvix exhibits
+/// problems that the user could address.
+
+#[derive(Debug)]
+pub enum WarningKind {}
+
+#[derive(Debug)]
+pub struct EvalWarning {
+    pub node: rnix::SyntaxNode,
+    pub kind: WarningKind,
+}