Fix warnings (causing CI failures) with new Rust beta.

- Panic messages must now be string literals (we used `format!()` in
  many places; `panic!()` can take format strings directly).
- Some dead enum options with EVEX encoding stuff in old x86 backend.
  This will go away soon and/or be moved to the new backend anyway, so
  let's silence the warning for now.
- A few other misc warnings.

cranelift/filetests/src/test_binemit.rs

@@ -31,7 +31,6 @@ pub fn subtest(parsed: &TestCommand) -> anyhow::Result<Box<dyn SubTest>> {
 
 /// Code sink that generates text.
 struct TextSink {
-    code_size: binemit::CodeOffset,
     offset: binemit::CodeOffset,
     text: String,
 }
@@ -40,7 +39,6 @@ impl TextSink {
     /// Create a new empty TextSink.
     pub fn new() -> Self {
         Self {
-            code_size: 0,
             offset: 0,
             text: String::new(),
         }
@@ -98,9 +96,7 @@ impl binemit::CodeSink for TextSink {
         write!(self.text, "{} ", code).unwrap();
     }
 
-    fn begin_jumptables(&mut self) {
-        self.code_size = self.offset
-    }
+    fn begin_jumptables(&mut self) {}
     fn begin_rodata(&mut self) {}
     fn end_codegen(&mut self) {}
     fn add_stack_map(