Debugging checks

[ndryden]

This adds various debugging aids, including sanity checks on inputs (closes #190) that are enabled when AL_DEBUG is set.

• Debug checks can be enabled without requiring a debug build.
• Some default-initializations and sanity checks for null communicators, to make debugging communicator issues easier.
• Support disabling Aluminum's signal handler. (In the future, this should be reworked.)
• Add a number of sanity checks for things like null buffers, invalid ranks, or overlapping buffers.