Add meta definition for bitcast.

This instruction uses two type variables: input and output. Make sure that our
parser can handle it. The output type variable annotation is mandatory.

Add a ValueTypeSet::example() method which is used to provide better diagnostics
for a missing type variable.

tests/parser/tiny.cton.ref

@@ -35,3 +35,9 @@ ebb0(vx0: f32, vx1: f32):
     v1 = fcmp uno, vx0, vx1
     v2 = fcmp lt, vx0, vx1
 }
+
+function bitcast(i32, f32) {
+ebb0(vx0: i32, vx1: f32):
+    v0 = bitcast.i8x4 vx0
+    v1 = bitcast.i32 vx1
+}