Expose extra USART config options and fix word length bug

[Keybinder]

1. Exposes tx_invert() and rx_invert() USART/serial configuration options, which correspond to setting the TXINV and RXINV bits in USART_CR2 respectively.
2. Exposes a wordlength_7() configuration option, which corresponds to having a word length of 7.
3. Fixes a mistake in the peripheral initialisation where the M[1:0] bits were set incorrectly for word lengths of 7 and 9.

[burrbull]

just for note. You can rewrite this more compact:

                usart.cr1().modify(|_, w| {
                    w.ue().set_bit();
                    w.te().set_bit();
                    w.re().set_bit();
                    w.m1().bit(config.wordlength == WordLength::DataBits7);
                    w.m0().bit(config.wordlength == WordLength::DataBits9);
                    w.pce().bit(config.parity != Parity::ParityNone);
                    w.ps().bit(config.parity == Parity::ParityOdd);
                    w.fifoen().bit(config.fifo_enable)
                });

[Keybinder]

Good to know, thanks. I just didn't want to change it too much from how it was originally.

[usbalbin]

Thank you for your contribution!

Would you mind rebasing over the latest changes where the clippy issue should be resolved?

[Keybinder]

Is this ok now?

[usbalbin]

Great, Thank you!