This commit adds the ability to conditionally compile tests. To
implement this, common source files needed to be moved to a subdirectory
`common`.
Additionally, this commit splits each `mat` template instantiation into
a separate source file. This enables the linker to discard unused
instantations. If each instantiation is placed in the same file and also
into the .iwram section, the linker will include every symbol in the
resulting binary, leading to an extremely high IWRAM usage. To introduce
this split, a new header file "mat_impl.hpp" was added containing
implementation details for the template instantiations. "mat_impl.hpp"
should ONLY be included by the source files containing explicit
instantiations of `mat`.
