chore: tidy up `ast` package

[DavePearce]

There is still a fair amount of tidying which can be done to this package. Specifically:

• (Expressions) These are currently treated as open structs in some places (e.g. Add), and closed structs in others (e.g. VariableAccess). It would be good to give each a constructor, and suitable accessors.
• (Names) Unsure whether PerspectiveName and FunctionName are really required. Perhaps could just use Name[T] directly. Also, could we merge Name[T] into VariableAccess[T] ?
• (Array Access) Should use VariableAccess for the array source, rather than rolling its own binding.