Preserve the function table explicitly (#1970)

The main gc pass of unused items in wasm-bindgen was accidentally
removing the function table because we weren't properly rooting it in
the auxiliary section which has a few ways that imports can reference
the function table via intrinsics and closures.

Closes #1967

crates/cli-support/src/js/mod.rs

@@ -2889,7 +2889,7 @@ impl<'a> Context<'a> {
     }
 
     fn export_function_table(&mut self) -> Result<String, Error> {
-        match self.module.tables.main_function_table()? {
+        match self.aux.function_table {
             Some(id) => Ok(self.export_name_of(id)),
             None => bail!("no function table found in module"),
         }