Expose Extensions during ser+de through ron::Options

[juntyr]

This PR is based on #281 and #339 (with a potential application for #334 as well).

1. It adds a new Options struct from which all ser+de utils methods can be called.
2. The existing utils methods XXX now use Options::default().XXX as their implementation.
3. The Options struct can be configured with a set of default RON extensions
   a) During deserialization default extensions are automatically enabled, i.e. do not have to be included in the RON.
   b) During serialization default extensions are automatically enabled and are not included in the output RON. Additional extensions enabled through the PrettyConfig which are not in the set of default extensions are still included in the output RON. Therefore, this PR supersedes Add output_extensions option to PrettyConfig #339.
   c) This now enables programs which always use a set of extensions to enable them programmatically and exclude them from the RON config files.
4. This PR is fully backwards compatible by adding one extra method each to the Serializer and Deserializer and not changing the default behaviour (i.e. no default extensions).
5. This PR also does a tiny bit of cleanup in the docs and Default impl for PrettyConfig.

• Add and update the documentation
• I've included my change in CHANGELOG.md

[juntyr]

@torkleyy Here's my initial attempt at the Option - I'm looking forward to your feedback!

[torkleyy]

Sorry for the late review. I like where this is going!

[juntyr]

Sorry for the late review. I like where this is going!

@torkleyy If you like the API as it is right now (and you give me the go-ahead), I'll start updating the documentation.

[torkleyy]

Looks great, go ahead 👍

[juntyr]

@torkleyy I've added documentation and the CHANGELOG entry. I also made the following changes to the Options API:

• Options::build() is now Options::default()
• both Serializer and Deserializer now take the Options in their new constructors instead of the default_extensions. This makes Options more obtrusive but makes it much easier to add new options to the Options, e.g. for Revert #329 and add to_string/writer_advanced functions instead #334

I've also made some minor code cleanup adjustments:

• moved the Position from parse to errors as it is exposed anyways (both in de and error and rustdoc was not showing them as the exact same struct)
• made ser::State private
• hid the doc for ser::Compound as it is an implementation detail but cannot be private
• simplified PrettyConfig's Default impl so that it isn't duplicated between serde and Default