reject duplicate fields

[birkenfeld]

In https://github.com/adamgreig/stm32-rs/pull/83#issuecomment-435731760 we found that duplicate fields in a register aren't rejected by svd2rust. (There are probably other duplicate conditions that could be caught, such as duplicate register names?)

I'll make the required changes, but I need to know if you want to handle this in svd-parser already, while reading the SVD, or in svd2rust, while generating the output?

[ryankurte]

Hey, thanks for the issue! I'm not sure where that should go tbqh, @adamgreig @japaric do you have any thoughts?

[burrbull]

In my view it is better to check this in svd-parser with ValidationLevel::Strict