Serial changes v3

[AndreySmirnov81]

Breaking changes

• Passing the Clock parameter to Serial by reference.
• Serial::usart1/2/3 -> Serial::new.
• Serial implements Write<WORD> and Read<WORD> for WORD simultaneously as u8 and u16.

Added

• Allow access to the Tx and Rx parts of the Serial without the need for splitting.
• Allow Serial reconfiguration by references to the Tx and Rx parts.
• Allow Serial release after splitting.

Instead of #414 and #409.

[AndreySmirnov81]

@burrbull, please review and merge
(the changes are equivalent to pr #414 but do not contain "garbage" commits)

[burrbull]

bors r+

[bors]

Build succeeded:

• ci (stm32f100, stable)
• ci (stm32f101, stable)
• ci (stm32f103, stable)
• ci (stm32f105, stable)
• ci (stm32f107, stable)
• Rustfmt