Gtirb

* Ubuntu 18 and gcc7 are no longer supported.

* Added support for floating-point numbers in AuxData.
* Disabled testing the Python API using `setup.py test`. The tests can still be
run using ctest or Python's unittest module.
* Improved the performance of the Python API's ByteBlock.references property.
* Fixed a pair of bugs in C++ support for variants in AuxData

* Added type annotations to python API and made them available in package.
* Removed install-python target.

* Added various symbolic expression attributes.
* Updated Java API

* Removed SymStackConst support
* Added variant (union) support for AuxData
* Removed address and size from modules
* Modified Module::findSections(string) to return a range instead of iterator
* Added IR::findSection(string) to C++ API

* Added offset helpers to Python API
* Better support for `std::byte` when working with `ByteInterval`
* Fixed a bug which manifested when serializing a big-endian IR