Macro matrix

macro_rules! matrix {
    ( $( $( $x:expr ),+ ) ;+ $(;)? ) => { ... };
    ( $( [ $( $x:expr ),* $(,)? ] ),+ $(,)? ) => { ... };
}

Create a full dense matrix from rows. Usage:

• matrix![ [a, b, c], [d, e, f], [g, h, i] ]
• matrix![ a, b, c; d, e, f; g, h, i ]