Refactor the API

[Yamakaky]

Currently, everything is reexported from lib.rs. I'd rather have a division per file format, with each module having links to the format spec, basic examples... For example:

DeBin -> bin::Deserialize
DeBinErr -> bin::Error
DeJsonTok -> json::Token

Additionally, free functions could be moved from the traits to the module:

SerJson::serialize_json -> json::serialize
TomlParser::parse -> toml::parse (with a new type/struct Document = HashMap<String, Toml>)

I can do a PR if you are OK with it, after my other PR are merged or rejected.