Add warning when we failed to emit padding at the start of a union

This currently triggers for the atsamd21 for these fields:

```
WARNING: field Some(Ident("pmux1_1")) has different offset 177 than its
union container 176
WARNING: field Some(Ident("pmux1_2")) has different offset 178 than its
union container 176
```

the issue there is that we're missing `derivedFrom` support for
registers, and once implemented, that issue goes away.

Author: wez

src/generate/peripheral.rs

@@ -297,6 +297,12 @@ fn register_or_cluster_block(
         let mut region_fields = Tokens::new();
 
         for reg_block_field in &region.fields {
+            if reg_block_field.offset != region.offset {
+                eprintln!("WARNING: field {:?} has different offset {} than its union container {}",
+                    reg_block_field.field.ident,
+                    reg_block_field.offset,
+                    region.offset);
+            }
             let comment = &format!(
                 "0x{:02x} - {}",
                 reg_block_field.offset,