Implement jump tables (#453)

* Add 'jump_table_entry' and 'indirect_jump' instructions.

* Update CodeSink to keep track of code size. Pretty up clif-util's disassembly output.

* Only disassemble the machine portion of output. Pretty print the read-only data after it.

* Update switch frontend code to use new br_table instruction w/ default.

lib/codegen/meta-python/base/legalize.py

@@ -306,10 +306,10 @@ widen.legalize(
 
 for int_ty in [types.i8, types.i16]:
     widen.legalize(
-        br_table.bind(int_ty)(x, y),
+        br_table.bind(int_ty)(x, y, z),
         Rtl(
             b << uextend.i32(x),
-            br_table(b, y),
+            br_table(b, y, z),
         )
     )