Add a compatibility option to always prefix enum values with their typename.

Previously the following spec:

status:
  type: string
  enum: [ success, failed ]

would be generated as:

type Status string

const (
    Success Status = "success"
    Failed  Status = "failed"
)

or in case of conflicts:

type Status string

const (
    StatusSuccess Status = "success"
    StatusFailed  Status = "failed"
)

This means if you start out without conflicts but introduce conflicts in a later version the generated code unexpectedly changes from under you and you potentially have to fix your code in many places.

When the new option compatibility.always-prefix-enum-values is set to true, the latter code is always generated.

Closes: #625