search graph: cache provisional results between fixpoint iterations

[lcnr]

will still go through it once to add some comments and to explain the general idea somewhere before merging this.

r? @nikomatsakis @compiler-errors

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

[lcnr]

closing this for now, rewrote this PR in a separate branch and am still not convinced it is sound. Will require some further thought until anything's ready to land here