refactor(aquavm): improve readability of ap merger and handler (#354)

At the moment `Ap` state contains vector of generations, although the ap instruction itself allow to specify only one stream. It was done to support several streams in a result position in a future. But the current realization allows to handle empty vector that was implemented to support states for scalars and it turned out (for more info see #326) that states for scalars aren't actually needed.

Closes #355.

crates/air-lib/interpreter-data/src/executed_state/impls.rs

@@ -79,9 +79,9 @@ impl ExecutedState {
 }
 
 impl ApResult {
-    pub fn new(res_gens: Vec<u32>) -> Self {
+    pub fn new(res_generation: u32) -> Self {
         Self {
-            res_generations: res_gens,
+            res_generations: vec![res_generation],
         }
     }
 }