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/src/ir/jumptable.rs

@@ -45,6 +45,11 @@ impl JumpTableData {
         self.table.len()
     }
 
+    /// Boolean that is false if the table has missing entries.
+    pub fn fully_dense(&self) -> bool {
+        self.holes == 0
+    }
+
     /// Set a table entry.
     ///
     /// The table will grow as needed to fit `idx`.