Generate event enums

[LegNeato]

The Go event code often has "enums" like:

type DynamoDBStreamViewType string

const (
	DynamoDBStreamViewTypeNewImage        DynamoDBStreamViewType = "NEW_IMAGE"          // the entire item, as it appeared after it was modified.
	DynamoDBStreamViewTypeOldImage        DynamoDBStreamViewType = "OLD_IMAGE"          // the entire item, as it appeared before it was modified.
	DynamoDBStreamViewTypeNewAndOldImages DynamoDBStreamViewType = "NEW_AND_OLD_IMAGES" // both the new and the old item images of the item.
	DynamoDBStreamViewTypeKeysOnly        DynamoDBStreamViewType = "KEYS_ONLY"          // only the key attributes of the modified item.
)

We should convert them to Rust enums.

[LegNeato]

The events crate has now moved to https://github.com/LegNeato/aws-lambda-events. Because of GitHub limitations (https://help.github.com/articles/transferring-an-issue-to-another-repository/) this issue is being closed rather than transferred.

If it is still relevant, please open an issue in the new repo.

Sorry for the churn!