Better Circular DMA support

[rudihorn]

This is my initial draft on PR on issue #242.

The example serial-dma-circ-len.rs shows how to use it in more detail, but the most important changes are demonstrate in the following snippet:

    let mut circ_buffer = rx.circ_read(buf);

    // wait until we have 5 bytes
    while circ_buffer.len() < 5 {}

    let mut dat = [0 as u8; 4];
    assert!(circ_buffer.read(&mut dat[..]) == 4);

One thing I have noticed is that currently a lot of the responsibility in setting up DMA is coded in the using module (e.g. in the serial module. I've introduced a setup function, so that it happens within the DMA module, and the setup within the serial module looks like:

                fn circ_read(self, buffer: &'static mut B) -> CircBuffer<u8, Self>{
                    let buffer = buffer.as_mut_slice();
                    let paddr = unsafe { &(*$USARTX::ptr()).dr as *const _ as u32 };

                    let mut buf = CircBuffer::new(buffer, self);
                    unsafe { buf.setup(paddr, Priority::MEDIUM) };
                    buf
                }

I have renamed the old CircBuffer behaviour renamed into e.g. CircDoubleBuffer.

I have only implemented this for the serial module so far.