fix getFile() for serialization

[Egor18]

Hi! I found out that filename from SourcePosition is lost during serialization (it's null). Although line number, column, etc. are present in deserialized model.
This happens because getFile() uses compilationUnit, which is null in deserialized model.
I fixed that by introducing field 'file' in SourcePosition.
Also, I provided some tests. Particularly, without this fix the following assertions fail:

assertTrue(type.getPosition().getFile().equals(typeFromFile.getPosition().getFile()));
....
assertTrue(elem1.getPosition().getFile().equals(elem2.getPosition().getFile()));

Really want this fix, because without it we can't get any filename from SourcePosition from deserialized model.

[monperrus]

OK for me.

[pvojtechovsky]

OK for me too

[surli]

Hi @Egor18 thanks for the fix! Could you just add a contract in comment of the test, then I'll merge the PR?

[Egor18]

Thanks!