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/mod.rs

@@ -62,5 +62,8 @@ pub type InstEncodings = SecondaryMap<Inst, isa::Encoding>;
 /// Code offsets for EBBs.
 pub type EbbOffsets = SecondaryMap<Ebb, binemit::CodeOffset>;
 
+/// Code offsets for Jump Tables.
+pub type JumpTableOffsets = SecondaryMap<JumpTable, binemit::CodeOffset>;
+
 /// Source locations for instructions.
 pub type SourceLocs = SecondaryMap<Inst, SourceLoc>;