Support Verify attribute on Vec<T>

[anthok]

Tried the following code below and wasn't able to use Verify on a Vec<>, it looks related to the change made for
a926c09.

#[nom(Verify="*test.len() > 0")]
 pub test: Vec<Test>

Error

consider giving this closure parameter an explicit type, where the type for type parameter `O2` is specified: `: &O2`

However, if writing the nom code manually and defining the closure type it will work.

#[nom(Parse = "{
        let parse_test = |i| {
            Test::parse(i)
        };
        verify(many0(complete(parse_test)), |test: &Vec<Test>| test.len() > 0)
    }")]
    pub test: Vec<Test>