UsartSpi Support

[armandas]

This is the continuation of the @CoolSlimbo's good work in #562. Closes #561.

I've made the following changes:

• Resolved the CS issue by adding a new constructor to Spi: new_from_usart
• Updated usart_spi-feedback example
• Fixed clippy warnings in the macro as well as some typos in the comments
• Address some of the remaining comments

I have run the usart_spi-feedback example on an Arduino Mega 2560 and confirmed that USART 1, 2 and 3 work.

[armandas]

Hmm, atmega1284p build fails. Seems like avr-device issue?

avr-device-0.5.4/src/devices/atmega2560/usart0/ubrr0.rs

    #[doc = "Writes raw bits to the register."]
    #[inline(always)]
    pub fn bits(&mut self, bits: u16) -> &mut Self {
        unsafe { self.0.bits(bits) };
        self
    }

avr-device-0.5.4/src/devices/atmega1284p/usart0/ubrr0.rs:

    #[doc = "Writes raw bits to the register."]
    #[inline(always)]
    pub unsafe fn bits(&mut self, bits: u16) -> &mut Self {
        self.0.bits(bits);
        self
    }

[armandas]

Turns out it was not so hard to fix the above issue.

[armandas]

@Rahix I think this PR should be good to review. I tried to address the comments in #562, but let me know if there are any changes needed.