There is a custom compiler error, hinting that the "debug", "compare"
and "rand" feature are required for compiling and running the tests.
However, this error was not displayed when running `cargo test` due to
other compilation errors taking precedence.
This is now avoided by just not compiling the integration tests when the
necessary features are not enabled.
This introduces functionality for creating strategies for matrices and
vectors, as well as an implementation of Arbitrary.
Strategies for the geometric types (Point3, Quaternion etc.) are not
currently part of this contribution.
The current strategy implementation for matrices has some limitations
that lead to suboptimal shrinking behavior. This is documented in the
module-level docs, with some additional comments in the code.
