This allows hiding the implementation details for how to represent logstash
config types that don't directly map to nix expressions, particularly floats,
hashes, and name-value pair sets with repeated names. Instead of setting
__type and value directly, the user now uses these convenience functions to
generate their logstash config.
Since the logstash config file seemed very similar to a nixexpr, I decided
to map directly from nixexprs to logstash configs. I didn't realize until
too far in that this solution was probably way over-engineered, but it
works.
