librustc: De-`@mut` `llargs` and `lllocals` in the function context
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