Made some code style improvements.
Added some text rendering debug code.
Cleaned up our CMake build file.
Made some small improvements to our CMake build file.
Switched to backward-cpp for printing stack traces.
Switched to a fork of backward-cpp.

Removes the old backward-cpp module, replacing it instead with a fork
that will contain extra fixes or changes.
Updated the backward-cpp submodule.

Includes new backward-cpp commits which fix various build warnings.
Added a custom font metrics utility class.
Made the line wrap guide depend on using a monospaced font.

Previously, even if the user was using a non-monospaced font, we would
render the line wrap guide. However, because each character was a
different length, the line wrap guide's position would be arbitrary and
unhelpful in this circumstance.
Fixed a bug where the wrap guide position was incorrect.

This commit updates the code which draws our line wrap guide to position
it correctly. This is done using some new font metrics functions.

Additionally, this adds some extra debugging code and unit tests which
help ensure that our wrap guide offset computations are done correctly.