111.13.00

- Eliminated the dependence of `Bignum` on `Re2`, and reduced dependence
  from `Core` to `Core_kernel`.
- Extended the rounding interface to bring it in line with int and float
  rounding.
- Improved the performance of `Bignum`'s binprot.

    `Bignum`'s binprot had been to just binprot the decimal string
    representation.  This is both slow to do and unnecessarily big in
    the majority of cases.  Did something better in the majority of
    cases and fell back to this representation in the exceptional case.

        $ ./inline_benchmarks_runner
        Estimated testing time 20s (2 benchmarks x 10s). Change using -quota SECS.
        ┌──────────────────────────────────────────────────────┬──────────┬───────────┬────────────┐
        │ Name                                                 │ Time/Run │   mWd/Run │ Percentage │
        ├──────────────────────────────────────────────────────┼──────────┼───────────┼────────────┤
        │ bignum0.ml:Stable:Bignum binprot roundtrip compact   │   7.87us │   490.00w │     32.88% │
        │ bignum0.ml:Stable:Bignum binprot roundtrip classic   │  23.94us │ 1_079.00w │    100.00% │
        └──────────────────────────────────────────────────────┴──────────┴───────────┴────────────┘