libextra: Stop using `@mut MemWriter` in the EBML module
librustc: De-`@mut` `break_ln` and `cont_ln` in liveness
librustc: De-`@mut` `used_unsafe` in the type context
librustc: De-`@mut` the named region map in the type context
librustc: Remove the unused `stmt_map` from the borrow checker
librustc: De-`@mut` `CStore::metas`
librustc: De-`@mut` `CStore::extern_mod_crate_map`
librustc: De-`@mut` `CStore::used_crate_sources`
librustc: De-`@mut` `CStore::used_libraries`
librustc: De-`@mut` `CStore::used_link_args`
librustc: De-`@mut` `cstore::CStore`
librustc: De-`@mut` `lints` in the session
librustc: De-`@mut` the `reachable` map
librustc: De-`@mut` the `type_abbrevs` table
librustc: De-`@mut` the borrow check's `loan_map`
librustc: De-`@mut` the `CollectFreevarsVisitor`
librustc: De-`@mut` the `freevar_map`
librustc: De-`@mut` `FunctionContext::llenv`
librustc: De-`@mut` `FunctionContext::llretptr`
librustc: De-`@mut` `FunctionContext::alloca_insert_pt`