Incompatible with rand 0.9+

[dengelt]

Using this crate with rand 0.9 or later (current version is 0.9.1) leads to a compile time error like this:

error[E0433]: failed to resolve: could not find `distributions` in `rand`
 --> src/lib.rs:7:10
  |
7 | #[derive(RandGen)]
  |          ^^^^^^^ could not find `distributions` in `rand`
  |
  = note: this error originates in the derive macro `RandGen` (in Nightly builds, run with -Z macro-backtrace for more info)

This is caused by rand 0.9.0 renaming some symbols:

• module rand::distributions was renamed to rand::distr
• distribution Standard was renamed to StandardUniform