feat(tvix/eval): add local identifier access

This makes basic `let ... in ...` statements work correctly. It does
not yet account for the call frames pushed into the VM during function
application.

Change-Id: I67155171daf1a43011b96716dd9d1ab04b27db33
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6190
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/opcode.rs

@@ -62,6 +62,9 @@ pub enum OpCode {
     // Type assertion operators
     OpAssertBool,
 
+    // Access local identifiers with statically known positions.
+    OpGetLocal(usize),
+
     // Close scopes while leaving their expression value around.
     OpCloseScope(usize), // number of locals to pop
 }