Make ItemId local to the BindgenContext.

[emilio]

See for example #204, where that caused an spurious name change.

[highfive]

Please make a comment here if you intend to work on this issue. Thank you!

[emilio]

We should create a new field in BindgenContext, called last_item_id, and use and increment that instead of the global atomic.

[emilio]

This was blocking stuff, so I just did it in #263