Inherit trusted attribute from parents

[xldenis]

Here's a rough prototype of what was proposed to solve #1067.

I need to actually add tests here, but it should cover the intended usecase.

There's a wrinkle in the form of trusted on traits and types also having different semantics: I don't think we want to inherit trusted on types from parent modules, it makes them opaque. Similarly we probably don't want to inherit trusted for trait declarations?