Noteworthy changes:
- All constructors now follow the same pattern: `new` uses OS entropy,
`from_rng` takes a `R: Rng` and seeds the randomness from there.
`from_rng` is fallible, since randomness generators can be fallible.
- `BalanceLayer` was renamed to `MakeBalanceLayer`, since it is not
_really_ a `BalanceLayer`. The name of `BalanceMake` was also
"normalized" to `MakeBalance`.
Another observation: the `Debug` bound on `Load::Metric` in
`p2c::Balance`, while not particularly onerous, generates really
confusing errors if you forget it include it. And crucially, the error
never points at `Debug` (should we file a compiler issue?), so I pretty
much had to guess my way to that being wrong in the doc example.
It would probably be useful to add a documentation example to
`MakeBalanceLayer` or `MakeBalance` (I suspect just one of them is fine,
since they're basically the same). Since I've never used it, and find it
hard to think of uses for it, it might be good if someone with more
experience with it wrote one.
1c2d50680a