LibJS: Give VM a cache of single-ASCII-character PrimitiveString

A large number of JS strings are a single ASCII character. This patch
adds a 128-entry cache for those strings to the VM. The cost of the
cache is 1536 byte of GC heap (all in same block) + 2304 bytes malloc.

This avoids a lot of GC heap allocations, and packing all of these
in the same heap block is nice for fragmentation as well.

Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -42,6 +42,10 @@ PrimitiveString* js_string(Heap& heap, String string)
 {
     if (string.is_empty())
         return &heap.vm().empty_string();
+
+    if (string.length() == 1 && (u8)string.characters()[0] < 0x80)
+        return &heap.vm().single_ascii_character_string(string.characters()[0]);
+
     return heap.allocate_without_global_object<PrimitiveString>(move(string));
 }