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.

cranelift/filetests/wasm/control.clif

@@ -48,7 +48,7 @@ ebb0:
 }
 
 function %br_table(i32) {
-jt0 = jump_table ebb3, ebb1, 0, ebb2
+jt0 = jump_table [ebb3, ebb1, ebb2]
 
 ebb0(v0: i32):
     br_table v0, ebb4, jt0