Fix: Map attributes in rust binding

[cbrzn]

This PR aims to fix an error on build when if we wanted to create an attribute that was a map, i.e:

type Data {
    total_market_cap: Map! @annotate(type: "Map<String!, BigNumber!>!")
}

it would fail on rust wrappers. I went ahead and removed the ? if the object is a map, and it fixes the issue. I also went ahead and added tests for this.