Remove the concept of non-dense jump tables.

WebAssembly doesn't have non-dense jump tables, and higher-level users
are better served by the facilities in lib/frontend/src/switch.rs for
working with non-dense switches.

This eliminates the concept of "absent" jump table entries, which
were represented as "0" in the text format.

Also, jump table contents are now enclosed in `[` and `]`, so that
we can unambiguously display empty jump tables. Previously, empty jump
tables were displayed as if they had a single absent entry.

lib/codegen/src/flowgraph.rs

@@ -129,8 +129,8 @@ impl ControlFlowGraph {
                     if let Some(dest) = dest {
                         self.add_edge(ebb, inst, dest);
                     }
-                    for (_, dest) in func.jump_tables[jt].entries() {
-                        self.add_edge(ebb, inst, dest);
+                    for dest in func.jump_tables[jt].iter() {
+                        self.add_edge(ebb, inst, *dest);
                     }
                 }
                 BranchInfo::NotABranch => {}