Switch back to svd2rust legacy identifier theme

[Rahix]

After starting work on porting avr-hal to the new version of avr-device, I started hitting the effects of the changed identifiers in the generated API. In Rahix/avr-hal#545, commit Rahix/avr-hal@ef69e1b, I at least got to the point of getting the Arduino Uno examples to compile again.

I have to say, I absolutely do not like the code churn we introduce with this. While it is probably manageable for avr-hal with team effort, that's not all:

• All downstream users will have to work through the same churn of (sometimes non-trivial) identifier changes.
• All current PRs in avr-hal will need to be rebased by their authors to the new style identifiers.
• Legacy examples found on the web all become outdated.
• Rust compiler error messages on fixing the names are quite hit-or-miss. The update strategy is not clear if you don't understand what has happened.

I took a look around the ecosystem and others have also decided to stick to what svd2rust calls the "legacy" identifier theme. I am much in favor of doing the same. There is no significant advantage to doing this change now, so let's not waste huge amounts of effor on it. We already break enough other things with this update as it stands...

Cc: @LuigiPiucco @tones111

[Rahix]

An additional advantage of switching back is that the special-casing magic of #194 / #193 will not be necessary then.