Toml serializer

[A248]

I have a dependent configuration library which supports multiple configuration formats depending on the caller's choice. Toml has been requested a few times, but I still need the ability to write comments on the toml values at serialization time. Unfortunately, TomlTable#entrySet does not use any kind of "node" type, which makes it harder to provide additional metadata to toml values.

In the original issue for toml serialization, #16, it was suggested to add a type hierarchy for toml values to enable us to write toml effectively. Such a suggestion could be worked into a version 2 of this library. Comments could be attached to the objects mentioned:

Exposing something similar to Jackson/Gson/Hjson/Jankson's type hierarchy (JsonElement, JsonObject, JsonPrimitive) would be great too.

[cleishm]

We'll use this issue for all things related to getting a complete Toml Serializer implementation.

While toToml() methods were added in #49, this is limited to re-serializing a previously parsed document. For a complete implementation, I suspect we'd want the following:

• APIs for constructing a Toml document model (e.g. a writable/modifiable version of TomlTable, etc).
• APIs for adding metadata (e.g. comments) to the document model.

Feel free to suggest anything else.