refactor(tvix/eval): return a lambda from the compiler

Changes the internal compiler plumbing to not just return a chunk of
code, but the same chunk wrapped inside of a lambda value.

This is one more step towards compiling runtime lambdas.

Change-Id: If0035f8e65a2970c5ae123fc068a2396e1d8fd72
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6240
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/chunk.rs

@@ -1,7 +1,7 @@
 use crate::opcode::{CodeIdx, ConstantIdx, OpCode};
 use crate::value::Value;
 
-#[derive(Debug, Default)]
+#[derive(Clone, Debug, Default)]
 pub struct Chunk {
     pub code: Vec<OpCode>,
     pub constants: Vec<Value>,