syntax: indicate an error when a macro ignores trailing tokens.

That is, only a single expression or item gets parsed, so if there are
any extra tokens (e.g. the start of another item/expression) the user
should be told, rather than silently dropping them.

An example:

    macro_rules! foo {
        () => {
            println("hi");
            println("bye);
        }
    }

would expand to just `println("hi")`, which is almost certainly not
what the programmer wanted.

Fixes #8012.