Implement something like choose_weighted for `Gen`

Can we include something like [frequency](https://hackage.haskell.org/package/QuickCheck-2.14.2/docs/Test-QuickCheck.html#v:frequency) from Haskell's QuickCheck? It will generalize [this pattern](https://github.com/BurntSushi/quickcheck/blob/master/src/arbitrary.rs#L632) and make it more convenient to use, for example:
```rust
g.choose_weighted(&[
(10, g.gen_range(0..0xB0) as u8 as char),
(2, ...),
(5, ...),
(1, ...),
(1, ...),
(1, ...),
])
```
The implementation can be something like this:
```rust
impl Gen {

pub fn choose_weighted<'a, T>(&mut self, slice: &'a [(u32, T)]) -> Option<&'a T> {
slice.choose_weighted(&mut self.rng, |item| item.0).ok().map(|item| &item.1)
}

}
```
I know that in this particular example can be problem with eager evaluation which is not problem in Haskell since it is lazy, however, we can solve it by adding another associated function which chooses from closures instead of values. In other words, do it exactly like it is done for `Result` with `and`/`and_then` and `or`/`or_else` functions.

What do you think?