Refactor VM evaluation to use macro

The `eval` and `eval_observed` methods in the `VM` struct were very
similar, with the primary difference being the call to the `VMObserver`
trait. This commit refactors these methods into a single macro,
`dispatch_eval!`, which reduces code duplication and improves
maintainability.

The `Value::TailCallRequest` variant was also updated to use `Box` for
its contents, which helps keep the `Value` enum size consistent.

Finally, the benchmarks in the `examples` directory have been updated to
reflect minor performance changes resulting from these code
modifications.

examples/closure.myc

@@ -1,5 +1,5 @@
 ;; Closure Scope Test
-;; Benchmark: 9.0us
+;; Benchmark: 9.4us
 ;; Output: 15
 (do
   (def make-adder (fn [x]