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/cretonne/src/ir/extname.rs

@@ -5,6 +5,7 @@
 //! Cretonne, which compiles functions independently.
 
 use std::fmt::{self, Write};
+#[allow(unused_imports)]
 use std::ascii::AsciiExt;
 
 /// The name of an external can be any sequence of bytes.

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};