refactor(tvix/eval): remove todo!() calls in compiler

It is impossible for tvixbolt to recover from panics, so the user
experience of typing an expression using an unsupported feature was
that it would get sad and stop responding to input.

Instead, raise a normal value-level error of a new variant and
continue where possible.

Change-Id: Ibe016c92cacb87b85095c0f83758eddc6468053e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6528
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/warnings.rs

@@ -9,7 +9,7 @@ pub enum WarningKind {
     ShadowedGlobal(&'static str),
 
     /// Tvix internal warning for features triggered by users that are
-    /// not actually implemented yet.
+    /// not actually implemented yet, but do not cause runtime failures.
     NotImplemented(&'static str),
 }