Module support

[Bromeon]

Adds support for mod and static.

Modules currently can contain these item types (which can now be top-level Declaration as well):

• const
• static (new)
• fn
• struct
• enum
• union
• impl

These item types can be inside mod, but are not parsed as top-level Declarations themselves:

• #![inner] attributes
• use statements

Not supported:

• Declarative macros
• Traits (will follow in a later PR)
• extern crate
• possibly some other token combinations I forgot

[Bromeon]

Thanks for the feedback! 🙂

Fixed the use parsing and removed the FIXME. Postponed unsafe mod, which I'm going to work on in an hour or so.

Added one commit 79b6312c4a15370323b532cf65b60c6e2deecde8; if you're fine with that, we can merge.