Gtirb

* Use GTIRB symbolic expression attributes.

* Access functions for converting to/from protobuf are no longer public in the C++ API.
* The proto library is no longer dllexported.
* GTIRB_EXPORT_API no longer uses dllimport on the client side.

* No longer installs Python files by default. Added a new 'install-python'
target to install Python files.

* Add preliminary ARM64 support.

* Moved protobuf definitions into gtirb.proto package (gtirb::proto namespace
in C++).
* Installing the Python API now respects CMAKE_INSTALL_PREFIX and DESTDIR with
their usual semantics.

* AuxData and AuxDataContainer in the C++ API have been reworked to provide cleaner type safety.
* AuxData is now retrieved directly from an AuxDataContainer using
a schema class that specifies both the name of the AuxData object
as well as its type.
* Schemata for AuxData types must be registered at process startup
before GTIRB objects are constructed or unserialized.