Make struct Name into an expression

[liquidev]

Declaring structs ahead of time is a necessity for supporting mutually recursive types, but most types aren't mutually recursive. Therefore it would make sense for struct Name to return the created type in addition to binding it to a variable Name.

Combined with Mica's flexible impl blocks, this would allow for the following idiom:

impl struct Vector
  func new(x, y) construct
    # ...
  end
end

[liquidev]

Considering my plan for implementing modules it would also make sense for impl blocks to return the implemented struct.