dex_utils: the xyk solver sometimes returns negative solutions

[erwanor]

dex_utils::xyk::solve computes the individual pool invariants of the x*y=k curve, it sometimes returns negative solutions which are nonsensical in this context. Instead, of failing when we encounter negative solutions, we should bake in the solver that we expect positive solutions, fail early if we detect a system to be non-convergent, or proceed otherwise.

[erwanor]

This can be sidestepped entirely during solving, which we will have to do as part of doing bounded liquidity for #4112. We're going to try using NNLS and see how far we can get.