Adding `next_u128` and `try_next_u128` to `Rng` and `TryRng` respectively.

[Apersoma]

Background

What is your motivation?

There not a compact way of randomly generating 128-bit integers with Rng and TryRng, unlike the built-ins for u32 and u64, or doing rng.next_rng() as u8/u16 for u8 and u16.

What type of application is this? (E.g. cryptography, game, numerical simulation)

Could be used by almost any application that uses 128-bit ints for something. I personally will be using this when generating test cases for mathematic functions on 128-bit integers.

Feature request

Self-explanatory. Implemented here: #78

[newpavlov]

The trait aims to provide a "fundamental" RNG interface on top of which other RNG stuff can be built. The trait provides next_u32/u64 in addition to fill_bytes because most PRNGs generate either u32 or u64, so it has to be exposed for efficiency reasons. AFAIK none of RNGs used in practice generate u128, so I believe the suggested method is out of the scope.