Add input buffer enable/disable support.

[GBJin]

Add a new function to set input buffer (enable/disable) for power saving. For Surface products, gpio configured as output pin should has input buffer disabled in deep doze.

[felipebalbi]

LGTM, but shouldn't every Output have input buffer disabled?

[kurtjd]

nit: This blank line is not necessary

[jerrysxie]

LGTM, but shouldn't every Output have input buffer disabled?

Output with input buffer disabled cannot read back the state of the pin as an input.

[jerrysxie]

We can already do this with Sense enabled and disabled state. If we add this API, this would break the typestate.

    /// Return a new Flex pin instance with level sensing disabled.
    ///
    /// Consumes less power than a flex pin with sensing enabled.
    #[must_use]
    pub fn disable_sensing(self) -> Flex<'d, SenseDisabled> {
        // Cloning the pin is ok since we consume self immediately
        let new_pin = unsafe { self.pin.clone_unchecked() };
        drop(self);
        Flex::<SenseDisabled>::new(new_pin)
    }

[felipebalbi]

LGTM, but shouldn't every Output have input buffer disabled?

Output with input buffer disabled cannot read back the state of the pin as an input.

when you try to read, grab a critical_section then enable input buffer, read, disable input buffer?

[jerrysxie]

LGTM, but shouldn't every Output have input buffer disabled?

Output with input buffer disabled cannot read back the state of the pin as an input.

when you try to read, grab a critical_section then enable input buffer, read, disable input buffer?

We could do that. Right now sensing is enforced with typestate. is_high() and is_low() only existed for Flex<'d, SenseEnabled>. If we want to make this more dynamic then we have to remove the typestate and returns an error when is_high() and is_low() are invoked with input buffer disabled.