Function bicgstab_solve 

pub fn bicgstab_solve(
    a: &CsrMatrix,
    b: &[f64],
    x0: &[f64],
    max_iter: usize,
    tol: f64,
) -> (Vec<f64>, usize)

BiCGSTAB solver for general (possibly non-symmetric) systems A x = b.

Returns (solution, iterations_used).