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/verifier/mod.rs

@@ -1229,8 +1229,8 @@ impl<'a> Verifier<'a> {
                         );
                     }
                 }
-                for (_, ebb) in self.func.jump_tables[table].entries() {
-                    let arg_count = self.func.dfg.num_ebb_params(ebb);
+                for ebb in self.func.jump_tables[table].iter() {
+                    let arg_count = self.func.dfg.num_ebb_params(*ebb);
                     if arg_count != 0 {
                         return nonfatal!(
                             errors,