Remove default generation of `@type t` for structs

`Struct{}` in typespecs will generate default types for all missing
fields.
Raise on multiple clauses with default arg
Merge pull request #2466 from ericmj/typespec-struct

Remove default generation of `@type t` for structs
Merge pull request #2467 from ericmj/multiclause-defaule

Raise on multiple clauses with default arg