compiler: export indexed type data, read unexported types lazily

    
    Introduce a new "types" command to the export data to record the
    number of types and the size of their export data. It is immediately
    followed by new "type" commands that can be indexed. Parse all the
    exported types immediately so that we register them, but parse other
    type data only as needed.
    
    Reviewed-on: https://go-review.googlesource.com/c/143022

From-SVN: r265409