Update 5.6 support: primitive alias

[NathanReb]

This PR adds support for 5.6 primitive alias feature.

In 5.6 it is now possible to define an alias on a primitive defined elsewhere. Primitives are now represented differently in the AST so it needs to be properly migrated in between 5.6 and 5.5 and for primitive aliases, properly encoded/decoded.

[NathanReb]

I'm opening as draft for now as it lacks the following:

1. encoding/decoding of primitive aliases
2. tests

In its current state though it builds on 5.6 and will correctly handle code that does not use primitive aliases so it might prove useful to people that want to give trunk a shot.

I'll add full support and tests in the next few days!

[NathanReb]

I'm also hoping to see the trunk build finally go green!