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/codegen/src/isa/x86/binemit.rs

@@ -277,6 +277,7 @@ impl EvexContext {
 }
 
 /// The EVEX format allows choosing a vector length in the `L'` and `L` bits; see `EvexContext`.
+#[allow(dead_code)]
 enum EvexVectorLength {
     V128,
     V256,
@@ -296,6 +297,7 @@ impl EvexVectorLength {
 }
 
 /// The EVEX format allows defining rounding control in the `L'` and `L` bits; see `EvexContext`.
+#[allow(dead_code)]
 enum EvexRoundingControl {
     RNE,
     RD,