Release Rx, Tx resource back to Pin

[lgjonathan]

Since the Serial Struct does not own the pins, I can't regain the ownership of the pins by calling release()
I was wondering if there's a workaround of this?

My use case is that the tx pin sometimes need to transfer data, but sometimes need to send a specific
amount of duration of ping.

I'd be happy to rewrite the serial module like stm32f4xx_hal if this feature requires a rewrite.
stm32f4xx-hal provides a way to release the pin ownership: stm32-rs/stm32f4xx-hal#748