Gtirb

* The Java API has been substantially reworked. Including:
* Most of the core API classes now have a more polished interface.
* The API for handling AuxData has been completely redesigned.
* New testing infrastructure based on JUnit5 has been added.
* More extensive test cases have been added.
* Add elfStackExec and elfStackSize AuxData definitions
* Add `IR.modules_named` helper method to Python API.
* Add `IR.findModules(String name)` helper method to Java API.

* Add elfDynamicInit and elfDynamicFini AuxData definitions

Note that this release, due to changes to the protobuf definitions of symbolic
expression attributes, is backwards-incompatible with previous GTIRB files.

* Replace symbolic expression attributes with composable labels.

* Added support for bool values in AuxData.
* Added elfSymbolVersions provisional AuxData definition.
* The GTIRB file format has changed to include a prefix containing a "magic"
identifier and the GTIRB protobuf version number to allow easier id of
GTIRB files. The change is not backwards compatible. Newer versions of
GTIRB will not be able to load older GTIRB files.
* Converted the decode mode from an arbitrary integer into a ProtoBuf enum.
This obviously breaks compatibility with older GTIRB files.

* Ubuntu 18 and gcc7 are no longer supported.