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/lib.rs

@@ -5,6 +5,7 @@ mod eval;
 mod opcode;
 mod value;
 mod vm;
+mod warnings;
 
 #[cfg(test)]
 mod tests;