Add an accessor for the function table

This commit adds an intrinsics to the `wasm_bindgen` crate which
accesses the `WebAssembly.Table` which is the function table of the
module. Eventually the thinking is that a module would import its own
function table via native wasm functionality (via `anyref` and such),
but until that's implemented let's add a binding for it ourselves!

Closes #1427

tests/wasm/api.js

@@ -55,3 +55,9 @@ exports.debug_values = () => ([
     () => (null),
     new Set(),
 ]);
+
+exports.assert_function_table = (x, i) => {
+  const rawWasm = require('wasm-bindgen-test_bg.js');
+  assert.ok(x instanceof WebAssembly.Table);
+  assert.strictEqual(x.get(i), rawWasm.function_table_lookup);
+};