- Adding support for explicit tag based discriminant for Rust Enum parsing for serde_yaml

[dchakrav-github]

There is being started as a draft PR as there is change needed in yaml_rust implementation for propagation of tags for sequence and mapping start events. The change in yaml_rust address chyh1990/yaml-rust#147 (pending PR request filed there. Currently, Cargo.toml points to the public fork where the fix exists. This makes PR local checkout, building and inspecting the work easier), #147, that allows for explicit tags to be used to map to the Enum.

Please note, this PR explicitly choose not to address serialization with explicit tag discriminators. It isn't clear what dependency the consumers of this crate's have on the output serialization format. Can follow up with a separate PR change for Serializer change. @dtolnay seeking your advice on compatibility here.

Summary of the change:

• Co-change needed in yaml_rust to propagate Tags as a part of Sequence/Mapping Start events to map and use for deserialization
• Fixed scalar mapping to delegate to conversions to handle other primitive types with explicit discriminant. This handles the case of

#[derive(Deserialize, PartialEq, Debug)]
    enum E {
        A(String),
        B(String),
        C(f32), // <-- this is new
}

when a call is being made for de::VariantAccess::newtype_variant it delegate correctly to get the value, see enum Message type in test_de.rs

• Added unit testing for use cases for passing happy case
• Added unit testing into error case for corner use case

[dtolnay]

As of 0.9, serde-yaml no longer depends on yaml-rust, so I'll close this version of the implementation. Thanks anyway for the draft PR! Please take a look at 0.9 and if there is remaining work you still need, let's handle that in a separate PR.