Add into_static for schema::Document

[obmarg]

I'm running into issues using schema::Document with strings. I have a
function that looks roughly like this:

fn load_schema(filename: String) -> schema::Document<'static, String> {
    let schema = std::fs::read_to_string(filename);

    graphql_parser::schema::parse_schema<String>(&schema).unwrap()
}

However this won't compile, because the generated Document has taken a
lifetime from schema which doesn't outlive the function.

I was looking around the library and saw that the query::Document has an
into_static function when using Strings which resolves this. This
duplicates that functionality for schema::Document.

[tailhook]

Merged. Thanks!

[obmarg]

Thanks for merging @tailhook - any idea when this might make it's way into a release?