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/isa/x86/legalize-br-table.clif

@@ -9,7 +9,9 @@ function u0:0(i64) system_v {
 ebb0(v0: i64):
     v1 = stack_addr.i64 ss0
     v2 = load.i8 v1
-    br_table v2, jt0
+    br_table v2, ebb2, jt0
+
+ebb2:
     jump ebb1
 
 ebb1: