Support contract annotations

[Nadrieril]

Contracts were just added as an experimental feature. We should translate them.

#![feature(contracts)]

struct Baz { baz: i32 }

#[core::contracts::requires(x.baz > 0)]
#[core::contracts::ensures(|ret| *ret > 50)]
fn tail(x: Baz) -> i32
{
    x.baz + 50
}