Add an encoding test for RISC-V.

Test that the generated encoding tables work as expected.

Change isa::Encoding into a struct with named fields so the recipe and bits can
be accessed.

src/libcretonne/isa/mod.rs

@@ -107,11 +107,28 @@ pub trait TargetIsa {
 /// encoding *bits*. The recipe determines the native instruction format and the mapping of
 /// operands to encoded bits. The encoding bits provide additional information to the recipe,
 /// typically parts of the opcode.
-pub struct Encoding(u16, u16);
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Encoding {
+    recipe: u16,
+    bits: u16,
+}
 
 impl Encoding {
     /// Create a new `Encoding` containing `(recipe, bits)`.
     pub fn new(recipe: u16, bits: u16) -> Encoding {
-        Encoding(recipe, bits)
+        Encoding {
+            recipe: recipe,
+            bits: bits,
+        }
+    }
+
+    /// Get the recipe number in this encoding.
+    pub fn recipe(self) -> usize {
+        self.recipe as usize
+    }
+
+    /// Get the recipe-specific encoding bits.
+    pub fn bits(self) -> u16 {
+        self.bits
     }
 }