Merge #467

467: Allow unused FieldReader r=therealprof a=couchand

I'm developing code that's general across AVR chips.  I'd like to depend on the shared parts of Rahix/avr-device (see Rahix/avr-device#44).  When compiling without any specific device feature enabled, there are no users of the `FieldReader::new` method.  This PR skips the lint for this case.

This is somewhat related to #427 but not exactly.

Co-authored-by: Andrew Dona-Couch <hi@andrewcou.ch>

src/generate/generic.rs

@@ -243,6 +243,7 @@ where
     U: Copy,
 {
     /// Creates a new instance of the reader.
+    #[allow(unused)]
     #[inline(always)]
     pub(crate) fn new(bits: U) -> Self {
         Self {