Enums with data / sum datatype / algebraic datatypes

This is a story that already exists is pivotal tracker.

Solidity should support sum datatype similar to how they are supported by rust.

```
// Enum holding additional data (marked union / sum datatype)
enum Commitment {
Hidden(bytes32 hash),
Revealed(uint value)
}

function reveal(Commitment storage self, uint _value, uint _nonce) {
if (self == Commitment.Hidden && self.Hidden.hash == sha3(_value, _nonce))
self = Commitment.Revealed(_value);
}
```

Comparison between enum types and enum field names (like `Commitment.Hidden`) is done based on the enum value only (data is ignored) but comparisons between enum variables is a deep comparison.

Because of the fact that references to the storage location of `self.Hidden.hash` can be kept, while `self` is switched to `Revealed`, the storage and memory locations of `Commitment.Hidden.hash` and `Commitment.Revealed.value` cannot overlap.

The issue names no source files, tests, or entry points. Start by reviewing how Solidity currently represents enums and compare the requested syntax and semantics with Rust; done requires an accepted design for storage, memory, comparisons, and state transitions.

Written by the indexing model from the issue text.