Modifying the default implementation for `MulAcc`.

[taketo1024]

I'd like to use sprs with num-bigint, num-rational etc. However, sprs's matrix multiplication requires that the scalar type implements MulAcc, which BigInt and Rational types do not suffice.

We can make these types automatically implement MulAcc, if we change the code in mul_acc.rs from the current

impl<N, A, B> MulAcc<A, B> for N
where
    N: Copy,
    B: Copy,
    A: num_traits::MulAdd<B, N, Output = N> + Copy,
{
    fn mul_acc(&mut self, a: &A, b: &B) {
        *self = a.mul_add(*b, *self);
    }
}

to

impl<N, A, B> MulAcc<A, B> for N
where
    for<'x> &'x A: Mul<&'x B, Output = N>,
    N: AddAssign<N> 
{
    fn mul_acc(&mut self, a: &A, b: &B) {
        self.add_assign(a * b);
    }
}

Is this a bad idea? (regarding performance?) Thanks.

[taketo1024]

I have pushed the modified version on my folked repo. https://github.com/taketo1024/sprs/tree/mul_acc_default. I will send a PR if this modification seems good.

[mulimoen]

That seems like a good idea. We can judge the performance impact once the PR is up

[taketo1024]

Created PR. Closing this issue. Thanks!