Suppress an unused-import warning on AsciiExt.

See

https://users.rust-lang.org/t/psa-dealing-with-warning-unused-import-std-ascii-asciiext-in-today-s-nightly/13726

for details.

lib/reader/src/lexer.rs

@@ -7,6 +7,7 @@
 
 use std::str::CharIndices;
 use std::u16;
+#[allow(unused_imports)]
 use std::ascii::AsciiExt;
 use cretonne::ir::types;
 use cretonne::ir::{Value, Ebb};