Define meta language to specify event vocabularies

[ueno]

Currently, event vocabularies are loosely defined as tables. We probably should be more strict about how they are used together in a formal language. For example, TLS context names and events may be written as:

scope tls {
  context handshake_client {
    attribute protocol_version u16
    attribute ciphersuite u16
    attribute ext::extended_master_secret bool
    context key_exchange {
      attribute group u16
    context sign {
      attribute signature_algorithm u16
    }
    context verify {
      attribute signature_algorithm u16
    }
  }
}