Switch to 2018 edition

See merge request !102

Most of this was generated by the absolutely extraordinary `cargo fix`
subcommand. There were still 2 errors and a few warnings to patch up,
but compared to the normal 600+ errors, I'd say the fixer did a damn
good job! I'm also amazed that I could still start the VM after this,
I half expected some kinds of runtime failure...

Fix dropping grants without unmapping if they have no file descriptor (such as those from anonymous maps)

Lock resources before checking if we need to clean them.

See merge request !101

See the comments deleted by this commit for more details as to how
the race condition could effect the system.

Use rustc-demangle in the stack traces

See merge request !100