message: Reorganize notes about breaking changes
diff: # Release 0.2.0 (pending)
  -- :warning: [`num-bigint` now requires rustc 1.15 or greater][23].
  -- :warning: [The crate now has a `std` feature, and won't build without it][46].
  - This is in preparation for someday supporting `#![no_std]` with `alloc`.
  -- :warning: [The `rand` and `serde` dependencies have been updated to 0.4 and 1.0][24]
  - respectively, and neither are enabled by default. The `rustc-serialize`
  - crate is no longer supported by `num-bigint`.
  -- :warning: [`Shr for BigInt` now rounds down][8] rather than toward zero,
  - matching the behavior of the primitive integers for negative values.
  -- :warning: [`ParseBigIntError` is now an opaque type][37].
  -- :warning: [The `big_digit` module is no longer public][38], nor are the
  - `BigDigit` and `DoubleBigDigit` types and `ZERO_BIG_DIGIT` constant that
  - were re-exported in the crate root. Public APIs which deal in digits, like
  - `BigUint::from_slice`, will now always be base-`u32`.
  +### Enhancements
  +
  - [`BigInt` and `BigUint` now implement `Product` and `Sum`][22] for iterators
  of any item that we can `Mul` and `Add`, respectively. For example, a
  factorial can now be simply: `let f: BigUint = (1u32..1000).product();`
  if sufficient compiler support is detected.
  - The release also includes other miscellaneous improvements to performance.
  -:warning: indicates a **breaking change**.
  +### Breaking Changes
  +
  +- [`num-bigint` now requires rustc 1.15 or greater][23].
  +- [The crate now has a `std` feature, and won't build without it][46]. This is
  + in preparation for someday supporting `#![no_std]` with `alloc`.
  +- [The `rand` and `serde` dependencies have been updated to 0.4 and 1.0][24]
  + respectively, and neither are enabled by default. The `rustc-serialize` crate
  + is no longer supported by `num-bigint`.
  +- [`Shr for BigInt` now rounds down][8] rather than toward zero, matching the
  + behavior of the primitive integers for negative values.
  +- [`ParseBigIntError` is now an opaque type][37].
  +- [The `big_digit` module is no longer public][38], nor are the `BigDigit` and
  + `DoubleBigDigit` types and `ZERO_BIG_DIGIT` constant that were re-exported in
  + the crate root. Public APIs which deal in digits, like `BigUint::from_slice`,
  + will now always be base-`u32`.
  **Contributors**: @clarcharr, @cuviper, @dodomorandi, @tspiteri
  