Rollup merge of rust-lang#33532 - jseyfried:mutable_lowering_context, r=nrc

Clean up `hir::lowering`

Clean up `hir::lowering`:
 - give lowering functions mutable access to the lowering context
 - refactor the `lower_*` functions and other functions that take a lowering context into methods
 - simplify the API that `hir::lowering` exposes to `driver`
 - other miscellaneous cleanups

r? @nrc

Author: Manishearth