Problem:
`v:starttime`, `:uptime` use a monotonic high-resolution timer. This
only works as long as the timer keeps running (if the computer is
suspended the timer is paused). This is somewhat unintuitive, and
doesn't match the behavior of the `uptime` shell command.
Solution:
Implement `os_realtime` to get the real time since the
epoch in nanoseconds.
Problem:
- The `ZR` feature makes it more obvious that we need some sort of flag so that
an `ExitPre` / `QuitPre` / `VimLeave` handler can handle restarts differently
than a normal exit. For example, it's common that users want `:mksession` on
restart, but perhaps not on a normal exit.
- Nvim has no way to report its "uptime".
Solution:
- Introduce `v:starttime`
- Introduce `v:exitreason`
Problem:
- Not obvious which _meta/ are generated and which should be edited
manually.
- The require guard (`error('Cannot require a meta file')`) is not
consistently present in all meta files.
Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.
