borrowck: consolidate `mut` suggestions This converts all of borrowck's `mut` suggestions to a new `mc::ImmutabilityBlame` API instead of the current mix of various hacks. Fixes #35937. Fixes #40823. Fixes #40859. cc @estebank r? @pnkfelix
borrowck: consolidate `mut` suggestions This converts all of borrowck's `mut` suggestions to a new `mc::ImmutabilityBlame` API instead of the current mix of various hacks. Fixes #35937. Fixes #40823. Fixes #40859. cc @estebank r? @pnkfelix