[otlib] Rust refactoring requests

[jwnrt]

We're currently undergoing refactoring of opentitan{lib,tool} and our other host-side Rust code. I'm creating this issue to collect requests.

[jwnrt]

• The SPI interface in opentitanlib has some methods that appear to be unused, such as set_voltage and get_max_transfer_count.
• Some interface traits have confusing names, e.g. Target for SPI and Bus for I2C.