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/binemit/memorysink.rs

@@ -32,6 +32,8 @@ use std::ptr::write_unaligned;
 pub struct MemoryCodeSink<'a> {
     data: *mut u8,
     offset: isize,
+    /// Size of the machine code portion of output
+    pub code_size: isize,
     relocs: &'a mut RelocSink,
     traps: &'a mut TrapSink,
 }
@@ -49,6 +51,7 @@ impl<'a> MemoryCodeSink<'a> {
         MemoryCodeSink {
             data,
             offset: 0,
+            code_size: 0,
             relocs,
             traps,
         }
@@ -131,6 +134,10 @@ impl<'a> CodeSink for MemoryCodeSink<'a> {
         let ofs = self.offset();
         self.traps.trap(ofs, srcloc, code);
     }
+
+    fn begin_rodata(&mut self) {
+        self.code_size = self.offset;
+    }
 }
 
 /// A `TrapSink` implementation that does nothing, which is convenient when