backward incompatible change for 64-bit indexing

[John W. Eaton]

Octave's binary file format does not handle 64-bit dimensions, so we
are limited to saving vectors with less than 2^31 elements.
Currently, the save_binary and read_binary functions in the
octave_value class hierarchy unconditionally read and write 32-bit
signed values for matrix dimensions.  The file format does have a
10-byte header to identify the file, currently it is either Octave-1-L
or Octave-1-B (for little- and big-endian).  This can change to
Octave-2- for files that have 64-bit matrix dimensions.

My proposal is to do the following:

  * For versions of Octave built with --enable-64:

    - write files with the Octave-2- format

    - read either type of file

  * For versions of Octave built without --enable-64:

    - continue to write files with Octave-1- format

    - attempt to read either type of file, only failing if a value
      that has dimensions that are too large to store given the limits
      of 32-bit integer dimensions.

Unfortunately, I see no good way to make this change in a backward
compatible way because the save_binary and load_binary funtions in the
derived octave_X_value classes will have to know whether they are
writing or reading version 2 files.  This means that after the change,
all octave_value types that are defined in C++ will have to change to
handle the new function signatures.

Comments?

jwe