Change `define-structure' macro to handle `named' option better,

Change `define-structure' macro to handle `named' option better,
allowing it to be a constant which is used as the tag.  This allows
redefinition of several structures in the runtime system, making them
fasdumpable.  Change handling of packages to attach a package to its
environment if that environment is not already attached to another
package.  Change the rep loop to show this package name when the
package is changed; also add command `pe' to return the current
package.