Prevent masking value variables with imports

[Disasm]

Fixes #2

[mciantyre]

I'm having trouble cherry-picking this commit onto #3. I needed extra {} throughout, changing patterns like

{ use $periph::$reg::$field::offset; offset }

into

{ use $periph::{$reg::$field::offset}; offset }

Why is that? Macros are weak in my Rust understanding.

[Disasm]

Hmm, interesting. I removed braces because they looked useless to me, but apparently they are required for something.

[Disasm]

Updated. Now it works with your tests.

[adamgreig]

It's a shame this update reduces the readability of the generated code, but it does nicely solve the shadowing of values, thanks! If it wasn't for rust-lang/rust#48067 we could just write $periph::$reg::$field::offset directly without using it, but alas.