Add internal_pull_down() method

[Windfisch]

PR #266 has added a internal_pull_up() function. This PR adds internal_pull_down() for symmetry reasons

[burrbull]

Change to self also this one then:

stm32f4xx-hal/src/gpio.rs

Line 303 in d1123d8

pub fn internal_pull_up(&mut self, on: bool) {

Is this relevant to other modes. Alternate<OpenDrain, A> for example?

[Windfisch]

I have done the opposite, using the pub fn (&mut self, on: bool) -> () signature everywhere, instead of the pointless "taking Self, returning Self", because in no case there was a change in the type state.

One might want to think about whether type-stating this pull-up/down resistor state, but for now, I'd leave it. If we do this, we can easily change it back with only minor modifications needed in user code. Also, no feature I am aware of actually requires the pins in one particular pullup/down state, to typestating it might actually cause more harm than good.

Does that look good to you?

[Windfisch]

Ignore what I wrote above. I will be doing it the other way round, as originally requested, using the "self-consume and returning Self", because this enables us to do things like

stm32f4xx-hal/examples/ssd1306-image.rs

Lines 39 to 48 in d1123d8

	let scl = gpiob
	.pb8
	.into_alternate()
	.internal_pull_up(true)
	.set_open_drain();
	let sda = gpiob
	.pb9
	.into_alternate()
	.internal_pull_up(true)
	.set_open_drain();

[burrbull]

Merge conflict

[Windfisch]

fixed (hopefully)

[Windfisch]

@burrbull do you have time to review and merge this O:)?

[burrbull]

@burrbull do you have time to review and merge this O:)?

LGTM, but, please rebase and squash instead of merge.

[Windfisch]

@burrbull there you go

[burrbull]

bors r+

[bors]

Build succeeded:

• ci (stm32f401, stable)
• ci (stm32f405, stable)
• ci (stm32f407, stable)
• ci (stm32f410, stable)
• ci (stm32f411, stable)
• ci (stm32f412, stable)
• ci (stm32f413, stable)
• ci (stm32f415, stable)
• ci (stm32f417, stable)
• ci (stm32f423, stable)
• ci (stm32f427, stable)
• ci (stm32f429, stable)
• ci (stm32f437, stable)
• ci (stm32f439, stable)
• ci (stm32f446, stable)
• ci (stm32f469, stable)
• ci (stm32f479, stable)
• Rustfmt