[Bug #20950]

ifunc proc has the ep allocated in the cfunc_proc_t which is the data of
the TypedData object. If an ifunc proc is duplicated, the ep points to
the ep of the source object. If the source object is freed, then the ep
of the duplicated object now points to a freed memory region. If we try
to use the ep we could crash.

For example, the following script crashes:

    p = { a: 1 }.to_proc
    100.times do
      p = p.dup
      GC.start
      p.call
    rescue ArgumentError
    end

This commit changes ifunc proc to also duplicate the ep when it is duplicated.

The hash value of a Proc must remain constant after a compaction, otherwise
it may not work as the key in a hash table.

The following code crashes with assertions enabled because envval could
be Qundef:

    {}.to_proc.dup