Propose to use CamelCase for SpiInterface structure

[luojia65]

I'd like to propose that we name the SPI interface structure SpiInterface other than SPIInterface, for that:

• in embedded-hal, Spi in SpiDevice and SpiBus are spelled in CamelCase, we can follow this convention to be consistent,
• in rust-lang Rust API guideline, it suggest that types are named in UpperCamelCase, with following descriptions:

In UpperCamelCase, acronyms and contractions of compound words count as one word: use Uuid rather than UUID, Usize rather than USize or Stdin rather than StdIn. In snake_case, acronyms and contractions are lower-cased: is_xid_start.

... in this case, SPI can be treated as an acronym, it counts as one word, so only the first 'S' should be upper cased.

r? @bugadani

[bugadani]

Thanks! Would you please update the other crates too, for consistency, and add a changelog entry (after rebasing, to the new Unreleased section)?

[luojia65]

@bugadani : I've pushed one commit (squashed) to include SpiInterface, I2cInterface and PGpioXBitInterface structures. r?

[bugadani]

Thanks!