Qualified imports

Fram supports only 2 types of imports: regular module imports and imports with full namespace opening. Whilst sufficient, this is not an elegant solution and often leads to either excessive boilerplate or namespace pollution. To counteract this problem, we can introduce quantified imports that allows user to manually select stuff to import. For example:
```
import Foo as {function1, Type2}
```
will only introduce two things, rather than full module.

Start by locating Fram's import parsing and name-resolution entry points, then trace how regular and full namespace-opening imports are handled. Define the behavior for the example `import Foo as {function1, Type2}` and add tests showing that only the selected names are introduced, including invalid or ambiguous selections.

Written by the indexing model from the issue text.