feat(tvix/eval): implement OpTailCall

If the last operation within a chunk is a function call, the call can
be executed in the same call frame without increasing the depth of the
call stack.

To enable this, a new OpTailCall instruction (similar to OpCall) is
introduced, but not yet emitted by the compiler.

Change-Id: I9ffbd7da6d2d6a8ec7a724646435dc6ee89712f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6457
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -104,6 +104,7 @@ pub enum OpCode {
 
     // Lambdas & closures
     OpCall,
+    OpTailCall,
     OpGetUpvalue(UpvalueIdx),
     OpClosure(ConstantIdx),