Be consistent with I2S and SPI terminology

[thejpster]

1. First we have to decide if using the legacy terminology is acceptable to us. Legacy terminology isn't acceptable to a subset of our audience, and I would strongly prefer we don't use it where there are reasonable alternatives.

2. Then we have to decide on suitable replacement terminology.

3. Then we have to go through the code base and ensure we use that replacement terminology consistently, and perhaps add one place where we clarify what the mapping of legacy to replacement terminology is, so anyone who searches for the legacy terminology finds the replacement terminology and knows what to search for next.

See https://en.wikipedia.org/wiki/Master%E2%80%93slave_(technology) for more background.

See also rust-embedded/embedded-hal#626 for a discussion around the embedded-hal traits.

I2C

Legacy	Replacement
Master	Controller
Slave	Target

SPI

Legacy	Replacement
Master	Controller
Slave	Peripheral
Peripheral	IP Block
MOSI	COPI
MISO	CIPO