112.06.00

- Sped up `String.is_substring` by replacing the OCaml implementation
with a call to libc `memmem`.

    `memmem` runs in 20% of the time, incurs minimal GC pressure, is
    portable among UNIXen that we target, AND it's clearer than the ML
    version.

- Made `Float_ref` support `bin_io` and `sexp`.
- Removed `gettid`, which is now available in `Core.Unix`.
- Added `Fast_int_div` module, which speeds up integer division by
  a fixed divisor.
- Moved `Sexp.of_sexp_allow_extra_fields` to core_kernel.