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.

cranelift/filetests/verifier/type_check.clif

@@ -71,10 +71,12 @@ function %jump_table_args() {
     jt1 = jump_table ebb1
     ebb0:
         v0 = iconst.i32 0
-        br_table v0, jt1 ; error: takes no arguments, but had target ebb1 with 1 arguments
-        return
+        br_table v0, ebb2, jt1 ; error: takes no arguments, but had target ebb1 with 1 arguments
+
     ebb1(v5: i32):
         return
+    ebb2:
+        return
 }
 
 function %jump_args() {