Auto merge of #69 - japaric:revise-api, r=japaric

revise peripheral API

This PR changes the signature of many of the high level methods available on peripherals like
`NVIC::get_priority`. Additionally some instance methods have been turned into static methods. The
following guidelines have been used to apply the changes:

- If the method body contains a single, atomic read operation with *no* side effects (e.g. the read
  operation clears one of the bits of the register): the signature changed to make the method
  static, i.e. `&self` was removed from the signature.

- If the method involves writing to or a RMW operation on a register: the signature changed to take
  the singleton by `&mut self` reference.

- If the method involves only read operations where at least one of them modifies the value
  of a register: the signature changed to take the singleton by `&mut self` reference.

The rationale for this last guideline is that using `&self`, instead of `&mut self`, lets the user
(unintentionally) break abstractions in the presence of generators. Example below:

``` rust
let peripherals = Peripherals::take().unwrap();
let syst = &peripherals.SYST;

// tasks
let mut a = || {
    loop {
        // yielding "busy wait"
        while !a.has_wrapped() {
            yield;
        }

        // do stuff
    }
};

let mut b = || {
    // ..

    // *NOTE* the problem is in the line below: this `is_counter_enabled` method reads the CSR
    // register and that read operation clears the COUNTFLAG bit of the register (if set), which is
    // the bit the `has_wrapped` method checks for.
    if syst.is_counter_enabled() {
        // ..
    }

    // ..
};
```

One more guideline was considered but the required conditions didn't apply to any of the existing
methods:

- If the method involves only non side effectful, non necessarily atomic read operations: the
  signature of the method should remain as `&self`.

The rationale for this guideline is that a static method (no `self` argument) wouldn't be
appropriate because that can result in a torn read if the read operation can be preempted by some
context that modifies the register.

In any case, this last guideline doesn't seem to apply well to the peripherals structs exposed by
this crate because they *deref* to a `RegisterBlock` that allows mutation through a `&self`
reference. When these two properties (the guideline and `Deref<Target=RegisterBlock>`) are mixed
the user can potentially break abstractions using generators (as shown in the `syst` example).

cc @hannobraun
closes #67