Serializing `[Event]`s ought not emit invalid YAML streams

[hvr]

See also https://github.com/haskell-hvr/HsYAML/pull/15#discussion_r289620891

We should have two modes: one which throws an exception when an invariant is broken by the stream; and one mode which silently fixes up the stream; this could either be a separate phase (i.e. a separate "stream transformer" function) or be integrated into the printing phase.