Introduce import statements

[sezna]

This PR introduces specific item imports, as opposed to our previous open-style imports which just glob-import an entire namespace.

For example:

import Foo.Bar;
import Foo.{Baz, Quux};
import Foo.{Bar.{Baz, Quux}, Corge as Grault};

[minestarks]

Where exactly should this error be thrown? I think technically, everything below the first open statement is a duplicate import, but I'm only getting errors for the last two.

open Bar;
import Bar.*;
import Bar.BarOperation;
import Bar.BarOperation;  // name error: imported symbol that already exists in scope
import BarOperation; // name error: imported symbol that already exists in scope

[sezna]

The first two are considered opens or glob imports, and are therefore "general". Item imports are "specific". Shadowing rules tend to prioritize specific over general, so globs are are shadowed by the item imports. That's why the following snippet works:

namespace Foo {
    operation F() : Unit {}
}
namespace Bar {
    operation F() : Unit {}
}
namespace Main {
    open Foo;
    import Bar.F;
    @EntryPoint()
    operation Main() : Unit {
        F()
    } 
}

This follows the behavior of Rust, and makes glob imports a bit more usable IMO. With that context, do you think we should change the behavior here?

[minestarks]

OK if this is intended shadowing behavior!