optional-type

[TheJJ]

It should be possible to declare types as optional(typename) which then allows values not to be set.

Currently, the specification states that whenever a nyan-object is used as value in another nyan-object, all the members must have an assigned value.

With this feature, members of the optional(othertype) type have a None value by default. They can be assigned None or a value of type othertype.

None becomes a keyword.

[TheJJ]

Done in #87, but None didn't become a keyword, rather it's a built-in now.