feat(tvix/eval): give generators human-readable names

This adds static strings to generator frames that describe the
generator in a human-readable fashion, which are then logged in
observers.

This makes runtime traces very precise, explaining exactly what is
being requested from where.

Change-Id: I695659a6bd0b7b0bdee75bc8049651f62b150e0c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8206
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/vm/macros.rs

@@ -48,7 +48,7 @@ macro_rules! cmp_op {
 
         let gen_span = $frame.current_light_span();
         $vm.push_call_frame($span, $frame);
-        $vm.enqueue_generator(gen_span, |co| compare(a, b, co));
+        $vm.enqueue_generator("compare", gen_span, |co| compare(a, b, co));
         return Ok(false);
     }};