112.06.00

- Renamed `Linux_ext.gettid` as `Unix.gettid`, and added OpenBSD support.

    `SYS_gettid` is not available on OpenBSD, but is used in
    `Core_extended`. See the mailing list discussion about this here:

    https://groups.google.com/forum/#!topic/ocaml-core/51knlnuJ8MM

    Seems like the OpenBSD alternative is:

        pid_t        getthrid(void);

    although it's not defined in any header file, which is a bit unfortunate.

- Added `Piecewise_linear.precache`, which computes a lookup table that
  speeds up subsequent calls to `Piecewise_linear.get`.
- Added `Time_ns` module, representing times as 63-bit integers of
  nanoseconds since the epoch.
- Fixed build of `unix_stubs.c` on OpenBSD.
- In `Daemon`, fixed an error message regarding `WSTOPPED` (fixes #47).
- Added `Time.Span.Stable.V2`, with sexps that use new suffixes for
  microseconds (`us`) and nanoseconds (`ns`).

    `Time.Span.of_string` supports the new format, but
    `Time.Span.to_string` doesn't yet produce it -- we plan to change
    that later, after the new `of_string` has made it out more widely.

- Added `Time.Span.to_string_hum`, which gives more options for
  rendering time spans.
- Merged the `recvmmsg` stubs in `Bigstring` and `Iobuf`.

    Factored out a shared underlying `recvmmsg` call that both
    stubs use.

    Restored `-pedantic` by avoiding a C99 feature (variable-length
    stack arrays).

- Made `Date.t` abstract, and changed its representation from a 4-word
  record to an immediate int (packing year, month, day).
- In `Daemon`, changed the permissions of the `std{err,out}` files
  generated during daemonization from `0o777` to `0o644`.
- Moved `Thread_safe_queue` from `core` to `core_kernel`.

    This was done so that `Async_kernel` can use it, eliminating one of
    `Async_kernel`'s dependencies on `Core`.

    `Thread_safe_queue_unit_tests` remains `Core`, at least for now,
    because it has some dependencies on other stuff in `Core`.