Mark panic_fmt as no_mangle to work around rust-lang/rust#38281 (#262)

src/lib.rs

@@ -86,6 +86,7 @@ extern "C" fn eh_personality() {}
 
 #[cfg(not(test))]
 #[lang = "panic_fmt"]
+#[no_mangle]
 extern "C" fn panic_fmt(fmt: core::fmt::Arguments, file: &'static str, line: u32) -> ! {
     println!("\n\nPANIC in {} at line {}:", file, line);
     println!("    {}", fmt);