Support user defined enums

[Joey9801]

Rust enum types are tagged unions. They don't have a fixed representation, eg the compiler optimises Option<&T> to elide the tag entirely, and reduces it to a single pointer where a NULL pointer represents the None variant.

The general case for generating bindings for arbitrary Rust enums is relatively involved. Several new structs need to be emitted + have binding metadata generated for them:

• A non-anonymous struct for each enum variant that holds data
• A C style union of each of those non-anonymous structs
• One final struct which contains an appropriately sized discriminator + an instance of the C style union.

Example:

#[dotnet_bindgen]
enum ExampleEnum {
    Foo,
    Bar(i32, i32),
    Baz { field_1: i32, field_2: i32 }
}

generates the following structs

#[repr(C)]
#[dotnet_bindgen]
pub struct __data_Bar (i32, i32)

#[repr(C)]
#[dotnet_bindgen]
pub struct __data_Baz {
    field_1: i32,
    field_2: i32,
}

#[repr(C)]
pub union __data {
    Bar: (i32, i32),
    Baz: __data_Baz,
}

struct __bindgen_enum_ExampleEnum {
    discriminator: u8,
    data: __data,
}

Normal binding metadata should be generated for each of the new explicitly named data variant structs, which can be easily accomplished by attaching the #[dotnet_bindgen] attribute to them.
The metadata emitted for the enumeration should include a mapping from discriminator value to variant.

TODO: The C# that should be generated for the above example.