New branch ready for review.

[John Darrington]

I have pushed a new branch called "data-encoding".  This branch
attempts to deal with character encoding issues more consistently.
In particular:

* Strings are stored internally as utf8, instead of converting to utf8
  on output.

* Conversion is done at the earliest possible stage; in the case of 
  variable names, labels and value labels, this means as they are read 
  from the .sav file (or other source); for string values, conversion is
  now done by the data_in/out functions.

* Per previous discussion, union values now have members of type uint8_t
  instead of char.  This provokes a compiler warning, if a value is being
  inappropriately forced into a string or vici-versa.  A few such warnings
  are still outstanding, but I think it?s safe to fix these later.

This branch is forked from "master".  Although I haven?t tried it, it should
be able to be merged with "output" and then non-ascii characters should
work properly in the output.

Comments about this branch are invited before I merge with master.

J?

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