grammar : coalesce find + insert into a single insert and adjust move/copy mechanics (#26885)

1) Combine two consecutive lookups (find + insert) into a single insert-attempt/lookup routine so that we don't per
form two O(log(n)) lookup operations in a row anymore -- we only need to do it once and then see if the insert succeeded.
2) Instead of copying every potential stack (expensive) and then moving it (cheap) to new_stacks when it's a final output state, we switch the order so that we move every potential stack (cheap), and then only copy it (expensive) to new stacks when it's a final output state. There are a LOT of intermediate states that get generated, and unless they become final output states, then all of these expensive intermediate copies are wasted.

Before: lookup -> lookup/insert + copy -> optional move to output
New: lookup/insert + move -> optional copy to output

src/llama-grammar.cpp

@@ -871,17 +871,18 @@ static void llama_grammar_advance_stack(
     std::set<llama_grammar_stack, decltype(stack_cmp)> seen(stack_cmp);
 
     while (!todo.empty()) {
-        llama_grammar_stack curr_stack = std::move(todo.back());
+        llama_grammar_stack curr_stack_candidate = std::move(todo.back());
         todo.pop_back();
 
-        if (seen.find( curr_stack) != seen.end()) {
+        auto [curr_stack_it, inserted] = seen.insert(std::move(curr_stack_candidate));
+        if (!inserted) {
             continue;
         }
-        seen.insert(curr_stack);
+        const llama_grammar_stack & curr_stack = *curr_stack_it;
 
         if (curr_stack.empty()) {
             if (std::find(new_stacks.begin(), new_stacks.end(), curr_stack) == new_stacks.end()) {
-                new_stacks.emplace_back(std::move(curr_stack));
+                new_stacks.emplace_back(curr_stack);
             }
             continue;
         }
@@ -924,7 +925,7 @@ static void llama_grammar_advance_stack(
         case LLAMA_GRETYPE_TOKEN_NOT:
             if (std::find(new_stacks.begin(), new_stacks.end(), curr_stack) == new_stacks.end()) {
                 // only add the stack if it's not a duplicate of one we already have
-                new_stacks.emplace_back(std::move(curr_stack));
+                new_stacks.emplace_back(curr_stack);
             }
             break;
         default: