Ignore clippy::cast_possible_truncation (pedantic)

“casting `usize` to `u8` may truncate the value”

https://rust-lang.github.io/rust-clippy/master/index.html#cast_possible_truncation

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/code_gen.rs

@@ -963,6 +963,7 @@ impl SourceGenerator {
 
         if conversion != ConversionFlag::None as usize {
             self.p("!")?;
+            #[allow(clippy::cast_possible_truncation)]
             let buf = &[conversion as u8];
             let c = std::str::from_utf8(buf).unwrap();
             self.p(c)?;