Add initial support for fat enums (internally-tagged representation)

[Veetaha]

Blocked by (these PRS have to reviewed and merged first before this one):

• Implement Attribute for FromAttributes + IntoAttributes #134
• Introduce #[dynomite(flatten)] attribute #132

Internally tagged enums

The docs are pretty self-explanatory. This PR adds support for simple fat enums that have either one-element-tuple variants of the nested value of type T such that T: Attributes, or unit variants.

use dynomite::Attributes;

#[derive(Attributes)]
#[dynomite(tag = "kind")] // required to specify for fat enums
enum Shape {
    Rectangle(Rectangle),
    // Use `rename` to change the **value** of the tag for a particular variant
    // by default the tag for a particular variant is the name of the variant verbatim
    #[dynomite(rename = "my_circle")]
    Circle(Circle),
    Unknown,
}

#[derive(Attributes)]
struct Circle {
    radius: u32,
}

#[derive(Attributes)]
struct Rectangle {
    width: u32,
    height: u32,
}

These Rust definition correspond to the following representation in DynamoDB for each enum variant:

• Rectangle:

  {
      "kind": "Rectangle",
      "width": 42,
      "height": 64
  }

• Circle:

  {
      "kind": "my_circle",
      "radius": 54
  }

• Unknown:

  {
      "kind": "Unknown"
  }

Closes: #131

How did you verify your change:

Added a unit test, trybuild tests and doc tests

What (if anything) would need to be called out in the CHANGELOG for the next release:

• Add initial support for fat enums (only one-element tuple variants of T: Attributes values and unit variants)

[Veetaha]

This should be ready for the review.
cc @softprops

[softprops]

@Veetaha I'll try to follow up within the next week. I'm getting caught up with some things at work but I'll try to chew through this iteratively

[Veetaha]

Friendly ping @softprops :D

[softprops]

Sorry about the delay. I've been caught up with a team transition at work. I just merged the last clean up pull and it seems there are now conflicts with these changes

[Veetaha]

Uh oh, that PR depended on this one. Now the changes from this PR are already in master ;d

[Veetaha]

We've been using our fork with this feature that we found lacking for us for quite some time successfully, I think it's fine to have this in master, so we are waiting for a new release on crates.io.