Change internal_pull_{up,down}() to "consuming self, returning Self".

This enables us to write ergonomic constructs like

```
let pin = gpiob
	.into_alternate()
	.internal_pull_up(true)
	.set_open_drain();
```

src/gpio.rs

@@ -300,14 +300,16 @@ impl<MODE, const P: char, const N: u8> Pin<Output<MODE>, P, N> {
 
 impl<const P: char, const N: u8> Pin<Output<OpenDrain>, P, N> {
  /// Enables / disables the internal pull up
- pub fn internal_pull_up(&mut self, on: bool) {
+ pub fn internal_pull_up(self, on: bool) -> Self {
  let offset = 2 * { N };
  let value = if on { 0b01 } else { 0b00 };
  unsafe {
  (*Gpio::<P>::ptr())
  .pupdr
  .modify(|r, w| w.bits((r.bits() & !(0b11 << offset)) | (value << offset)))
  };
+
+ self
  }
 
  /// Enables / disables the internal pull down