[DGD] DGD 1.6 and 1.5.6

[Felix A. Croes]

Today I committed some changes that I had been preparing for a while,
and released two new versions of DGD.

DGD 1.5.6 is a release on the new 1.5-stable branch.  This branch will
get only bugfixes, no new features.  I will keep it up to date for at
least a few years.

DGD 1.6 is a new release with some fairly radical changes.  Most importantly,
it abandons snapshot compatibility with any version before 1.5.6.  While I
liked having more than 20 years of snapshot compatibility, the maintenance
overhead for having all the compatibility functionality in the code, including
a different LPC VM for which there no longer is a compiler, grew too large.

To upgrade a snapshot to DGD 1.6, first upgrade to DGD 1.5.6 (the last
commit shared between the two branches is fine too), recompile all objects,
and create a full snapshot.  This snapshot will be readable by DGD 1.6.
Note that a snapshot created by DGD 1.6 will <not> be readable by DGD 1.5.6.

DGD 1.6 also throws out the network extensions, another piece of code with
an ever increasing maintenance overhead.  I look forward to welcoming
back a new version of the network extensions in the future, after it has
been brought more in line with the current state of DGD.  Anyone who
depends on the network extensions should switch to the 1.5-stable branch
for now.

Regards,
Felix Croes