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]

Not sure why but I'm getting the wrong span on this error in practice.

(Running the alex/glob-imports branch so not 100% sure which PR the bug is coming from).

[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

[minestarks]

Something about the asymmetry between the import and export syntaxes bothers me.

import Foo; // doesn't allow multiple items, allows globs, no braces required
export { Foo, Bar }; // allows multiple items, doesn't allow globs, braces required

In practice, it took me a few tries to arrive at the correct syntax as I was typing in the editor - knowing the export syntax was no help in figuring out the import syntax. Is there an opportunity to bring these a little closer together?

[minestarks]

I think there may be a problem with newtype imports:

namespace Foo {
    import Bar.NewType; // no error
    
    operation FooOperation() : Unit {
        let x: NewType = NewType("a");  // The constructor name is resolved, but the type name isn't
//             ^^^^^^^  name error: `NewType` not found
    }
}

namespace Bar {
    export { NewType }; // not sure if this is required, but it doesn't seem to matter
    newtype NewType = String;
}