Merge: lib: intro the matrix package

This PR intro `Matrix` to represent a matrix of floats. The main module offers only general services and the `projection` module adds services useful for 3D manipulations.

The package is oriented towards matrices of floats and was designed alongside Gamnit. As such, is a bit specialized and may not be appropriate for all usage of matrices (such as integer matrices). So if you prefer, I can move these back to Gamnit. But since this module does not have any dependencies and `Matrix` can be useful for a wide array of context, I chose to leave it in lib.

Pull-Request: #1719
Reviewed-by: Jean Privat <jean@pryen.org>