Feature: Enum Support

[du-toit]

This PR introduces encoding and decoding support for enum types and their variants.

Implementation

Consider this enum:

enum Enum {
    Unit,
    NewType(i32),
    Tuple(i32, i32),
    Struct { a: i32, b: i32 }
}

Each variant is encoded as such:

• Enum::Unit => 4:Unit.
• Enum::NewType(50) => d 7:NewType i50e e.
• Enum::Tuple(25, 50) => d 5:Tuple l i25e i50e e e
• Enum::Struct { a: 25, b: 50 } => d 6:Struct d 1:a i25e 1:b i50e e e

Note that I only added whitespace to the examples above so they are more readable.

I believe these changes are backwards-compatible, since no items were added or removed from the API. Because of this, I've bumped the package version to 0.5.4 and updated the readme accordingly.

Upon a successful merge, this PR closes #2.