Free form legacy support (Fortran90Legacy mode).

[dorchard]

This PR adds support for legacy extensions but in free form lexing, i.e., for Fortran 90 code. This handles issue #307

• new Fortran90Legacy version (and associated changes)
• Handle structure
• union
• map
• Handle automatic
• Handle static
• Tests pass

Note this can't handle structures with comments at the end of lines:

structure /foo/
  ! comment before union"
    union
      ! comment inside union, before map
         map
         ! comment inside map
             integer i ! more comment"
             end map
     end union
end structure

Does it need to?

[nikolaushuber]

LGTM