Constant rustc_trait_selection::solve::FIXPOINT_STEP_LIMIT

const FIXPOINT_STEP_LIMIT: usize = 8;

How many fixpoint iterations we should attempt inside of the solver before bailing with overflow.

We previously used tcx.recursion_limit().0.checked_ilog2().unwrap_or(0) for this. However, it feels unlikely that uncreasing the recursion limit by a power of two to get one more itereation is every useful or desirable. We now instead used a constant here. If there ever ends up some use-cases where a bigger number of fixpoint iterations is required, we can add a new attribute for that or revert this to be dependant on the recursion limit again. However, this feels very unlikely.