Make js.Bridge aware of string.String for input parameters

Avoids having to allocate small strings when going from v8 -> Zig. Also
added a discriminatory type, string.Global which uses the arena, rather than
the call_arena, if an allocation _is_ necessary. (This is similar to a feature
we had before, but was lost in zigdom). Strings from v8 that need to be
persisted, can be allocated directly v8 -> arena, rather than v8 -> call_arena
-> arena.

I think there are a lot of places where we should use string.String - where
strings are expected to be short (e.g. attribute names). But started with just
document.querySelector and querySelectorAll.

src/http/Http.zig

@@ -271,17 +271,21 @@ pub const Header = struct {
 };
 
 pub const Headers = struct {
-    headers: *c.curl_slist,
+    headers: ?*c.curl_slist,
     cookies: ?[*c]const u8,
 
     pub fn init(user_agent: [:0]const u8) !Headers {
         const header_list = c.curl_slist_append(null, user_agent);
-        if (header_list == null) return error.OutOfMemory;
+        if (header_list == null) {
+            return error.OutOfMemory;
+        }
         return .{ .headers = header_list, .cookies = null };
     }
 
     pub fn deinit(self: *const Headers) void {
-        c.curl_slist_free_all(self.headers);
+        if (self.headers) |hdr| {
+            c.curl_slist_free_all(hdr);
+        }
     }
 
     pub fn add(self: *Headers, header: [*c]const u8) !void {