refactor(tvix/eval): always pass slot to compiler methods

The slot is now always known (at the root of the file it is simply
stack slot 0 once the scope drops back down to 0), so it does not need
to be wrapped in an `Option` and accessed in cumbersome ways anymore.

Change-Id: I46bf67a4cf5cb96e4874dffd0e3fb07c551d44f0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6420
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/compiler/scope.rs

@@ -104,6 +104,10 @@ pub struct Upvalue {
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd)]
 pub struct LocalIdx(usize);
 
+impl LocalIdx {
+    pub const ZERO: LocalIdx = LocalIdx(0);
+}
+
 /// Represents a scope known during compilation, which can be resolved
 /// directly to stack indices.
 ///