Reduce allocation in IDRS inner loop

[samuelpowell]

This reduces allocations in the IDRS inner loop, ref #186.

• I am unfamiliar with the algorithm and have thus made minimum changes.
• I am unsure of the reason for splatting the operator into the linsys_op function. Tests pass but perhaps I've misunderstood this implementation choice.

[mschauer]

The design comes from a time where https://github.com/Jutho/LinearMaps.jl was not yet a thing and I wanted solve Lyapunov type equations. See https://github.com/mschauer/IDRsSolver.jl/blob/master/src/IDRsSolver.jl#L47 . Aligning the code with the other solvers is a good idea.

[haampie]

Looks good to me! The splatting shouldn't be necessary when LinearMaps is used.

[samuelpowell]

@mschauer ahh yes, that makes sense. Indeed, LinearMaps.jl is incredibly useful for this kind of thing.

@haampie thanks for review.