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/reader/src/sourcemap.rs

@@ -218,7 +218,7 @@ mod tests {
         let tf = parse_test(
             "function %detail() {
                                ss10 = incoming_arg 13
-                               jt10 = jump_table ebb0
+                               jt10 = jump_table [ebb0]
                              ebb0(v4: i32, v7: i32):
                                v10 = iadd v4, v7
                              }",