Generate instruction unwrapping code for binemit recipes.

Generate code to:

- Unwrap the instruction and generate an error if the instruction format
  doesn't match the recipe.
- Look up the value locations of register and stack arguments.

The recipe_* functions in the ISA binemit modules now take these
unwrapped items as arguments.

Also add an optional `emit` argument to the EncRecipe constructor which
makes it possible to provide inline Rust code snippets for code
emission. This requires a lot less boilerplate than recipe_* functions.

lib/cretonne/src/ir/valueloc.rs

@@ -33,6 +33,14 @@ impl ValueLoc {
         }
     }
 
+    /// Get the stack slot of this location, or panic.
+    pub fn unwrap_stack(self) -> StackSlot {
+        match self {
+            ValueLoc::Stack(ss) => ss,
+            _ => panic!("Expected stack slot: {:?}", self),
+        }
+    }
+
     /// Return an object that can display this value location, using the register info from the
     /// target ISA.
     pub fn display<'a, R: Into<Option<&'a RegInfo>>>(self, regs: R) -> DisplayValueLoc<'a> {