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/wasm/control.clif

@@ -51,7 +51,9 @@ function %br_table(i32) {
 jt0 = jump_table ebb3, ebb1, 0, ebb2
 
 ebb0(v0: i32):
-    br_table v0, jt0
+    br_table v0, ebb4, jt0
+
+ebb4:
     trap oob
 
 ebb1: