Fix Map#keys and Map#values returns (AssemblyScript#1004)

Author: LiaoPeng

std/assembly/map.ts

@@ -210,8 +210,7 @@ export class Map<K,V> {
     for (let i = 0; i < size; ++i) {
       let entry = changetype<MapEntry<K,V>>(start + <usize>i * ENTRY_SIZE<K,V>());
       if (!(entry.taggedNext & EMPTY)) {
-        keys.push(entry.key);
-        ++length;
+        keys[length++] = entry.key;
       }
     }
     keys.length = length;
@@ -227,8 +226,7 @@ export class Map<K,V> {
     for (let i = 0; i < size; ++i) {
       let entry = changetype<MapEntry<K,V>>(start + <usize>i * ENTRY_SIZE<K,V>());
       if (!(entry.taggedNext & EMPTY)) {
-        values.push(entry.value);
-        ++length;
+        values[length++] = entry.value;
       }
     }
     values.length = length;

std/assembly/set.ts

@@ -187,11 +187,9 @@ export class Set<T> {
     for (let i = 0; i < size; ++i) {
       let entry = changetype<SetEntry<T>>(start + <usize>i * ENTRY_SIZE<T>());
       if (!(entry.taggedNext & EMPTY)) {
-        values.push(entry.key);
-        ++length;
+        values[length++] = entry.key;
       }
     }
-    values.length = length;
     return values;
   }