remove the borrow_mut method from resources

it can be used to break references rules within nested locks

``` rust
static R1: Resource<bool, C1> = unsafe { Resource::new(false) };
static R2: Resource<bool, C2> = unsafe { Resource::new(false) };
static R3: Resource<bool, C3> = unsafe { Resource::new(false) };

// Priority = 1
extern "C" fn j1(task: interrupt::Exti0Irq) {
    R1.lock(&task, |r1, c1| {
        R2.lock(&task, |r2, c2| {
            R3.lock(&task, |r3, mut c3| {
                // BAD &- and &mut - that point to the same data
                let r1_ref: &bool = R1.borrow(&c2);
                let r1_ref_mut: &mut bool = R1.borrow_mut(&mut c3);
            });
        });
    });
}
```

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