feat(tvix/eval): Add Compiler::thunk method for emitting thunks

The logic in this method is *very* similar to `compile_lambda`. It is
intended to be called around any expression that should be
thunked (such as function applications, attribute set values, etc.).

Change-Id: Idfbb2daa9f4b735095378fb9c39a2fd07c8cff91
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6344
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -107,6 +107,9 @@ pub enum OpCode {
     OpGetUpvalue(UpvalueIdx),
     OpClosure(ConstantIdx),
 
+    // Thunks
+    OpThunk(ConstantIdx),
+
     /// Finalise initialisation of the upvalues of the value in the
     /// given stack index after the scope is fully bound.
     OpFinalise(StackIdx),