Move the call_arena to the page.

The call_arena was previously owned by the js.Context, but it has to exist on
the page, and the page is created before the context, so it's set to undefined
on the page. While this has never caused an issue, there's no reason for the
page not to own this, and the context to simply reference it.

Also, renamed the js.Context.context_arena to simply `arena`, which is more
consistent with other arena names (e.g. page.arena).

src/http/Http.zig

@@ -443,7 +443,6 @@ const LineWriter = struct {
     }
 };
 
-
 pub fn debugCallback(_: *c.CURL, msg_type: c.curl_infotype, raw: [*c]u8, len: usize, _: *anyopaque) callconv(.c) void {
     const data = raw[0..len];
     switch (msg_type) {