librustc: Implement `|A| -> B` syntax for closures and make bare `fn`
work
libstd: Remove mocks.
Make managed_boxes feature gate error less opinionated
auto merge of #10181 : brson/rust/managed_boxes, r=thestinger
auto merge of #10187 : pcwalton/rust/remove-mocks, r=pcwalton

r? @alexcrichton