Serialize views of data

Dynamically select during serialization which fields will be included.

Example

```rust use serdeview::View; use serdeview::ViewFields;

[derive(Clone, Debug, serde::Deserialize, serde::Serialize, View)]

pub struct MyStruct { id: String, #[serde(default)] name: String, #[serde(default)] tags: Vec, }

fn serialize(my: &MyStruct) -> Result { serdejson::tovalue(my.asview().withfields([ ::Fields::Id, ::Fields::Name, ])) } ```