Consider combining `SystemParam::validate_params` and `get_params`

[alice-i-cecile]

@cart says:

The redundant work in validate_param, which is done immediately before get_param never felt great to me. This isn't a problem exclusive to Res/ResMut, but it is certainly present.

I think doing validation as part of get_param is more defensible from a performance perspective.

I think it is worth doing some benchmarks where we skip validation (as-in, across all SystemParams), just to see what price we're paying here.

IMO the best way to restructure this would be to have get_param return a Result<Self::Item, SystemParamValidationError>, effectively combining the signature with validate_param.