Remove Sized constraint for generic Rng

[nvzqz]

Preface: This is a breaking change!!

Recently I tried using Rand on a Rng trait object but I was unable to since Rand::rand requires that the Rng be Sized.

This is mostly a proof-of-concept that should be considered when going through with the next iteration of this project.

Limitations:

• Methods like choose are implemented generically on Rng. These cannot be used within the context of a trait object.
• Cannot use conveniences like if rng.gen() { ... } when Rng is not Sized
• Breaks current #[derive(Rand)] implementation
  • Relies on gen and gen_range

[dhardy]

Think I already addressed this in my PR #161 (PR still needs a lot of work in general though).

[dhardy]

Implemented as part of #265 and #256.