[meta] Add Instruction helpers and change its representation to make it easily copiable;

- adds helpers used by other parts of the code
- allows cheap copies by having Instruction be a lightweight ref-cloned
wrapper of the actual instruction's content.

cranelift/codegen/meta/src/cdsl/operands.rs

@@ -134,7 +134,7 @@ pub struct OperandKind {
     /// The camel-cased name of an operand kind is also the Rust type used to represent it.
     pub rust_type: String,
 
-    fields: OperandKindFields,
+    pub fields: OperandKindFields,
 }
 
 impl OperandKind {