Support lifetime bounds in where clauses

[taiki-e]

Refs: #7

pin-project-lite/tests/test.rs

Lines 67 to 80 in 9ed0869

	// TODO(#7): where clause does not support yet.
	// pin_project! {
	// pub struct Struct3<T>
	// where
	// T: 'static,
	// {
	// field: T,
	// }
	// }
	// trait Static: 'static {}
	// impl<T> Static for Struct3<T> {}