feat(tvix/eval): initial attempt at setting lambda names

When compiling a lambda, take the name of the outer slot (if
available) and store it as the name on the lambda.

These names are then shown in the observer, and nowhere else (so far).

It is of course common for these things to thread through many
different context levels (e.g. `f = a: b: c: ...`), in this setup only
the outermost closure or thunk gains the name, but it's better than
nothing.

Change-Id: I681ba74e624f2b9e7a147144a27acf364fe6ccc7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7065
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/scope.rs

@@ -15,6 +15,8 @@ use std::{
     ops::Index,
 };
 
+use smol_str::SmolStr;
+
 use crate::opcode::{StackIdx, UpvalueIdx};
 
 #[derive(Debug)]
@@ -71,6 +73,14 @@ impl Local {
         }
     }
 
+    /// Retrieve the name of the given local (if available).
+    pub fn name(&self) -> Option<SmolStr> {
+        match &self.name {
+            LocalName::Phantom => None,
+            LocalName::Ident(name) => Some(SmolStr::new(name)),
+        }
+    }
+
     /// Is this local intentionally ignored? (i.e. name starts with `_`)
     pub fn is_ignored(&self) -> bool {
         match &self.name {