Split off ContextOps from Context

[leoyvens]

Remove it as a super trait, leaving ContextOps as a completely separate trait.

The motivation is that in rustc the ChalkContext which impls ContextOps carries an actual TyCtx<'a, 'tcx, 'gcx> and needs the extra 'a lifetime parameter. However types that take just Context need at most 'tcx and 'gcx, so to prevent the extra lifetime from leaking everywhere ContextOps is no longer a super trait of Context.

[nikomatsakis]

Thanks @leodasvacas !