Upgrade to Navi 0.6.0

[RunDevelopment]

Changes:

• Upgrade to navi@0.6.0
• Removed unnecessary int casts. The improved arithmetic in 0.6.0 means that Navi doesn't return +-inf anywhere like before.
• Removed toJson. The function was unused.
• Account for scopes. Builtin functions now always get the current scope as the first argument.
• Changing handling of structs. 0.6.0 significantly changed the layout of structs, so I had to change a lot of code.

About how structs changes: Structs kinds were previously differentiated by their name. E.g. the Image struct was a different kind than the Color struct. Instead of using the name, structs now have a descriptor that identifies the kind. Aside from the name, the descriptor also contains the field names, field definition types, and the default instance of the type (= all fields being the definition type).