Serialization refactor

[greg-szabo]

The serializers library started to become a bit unruly:

• The naming convention is not standardized
• Deserializer functions for Option<T> versions of different types were created when not necessary

The serializers library should focus on implementing custom ser/de for primitives and complex types that are not implemented by the source code. Things like i64 to String or time::Duration`` to a String that holds the duration in nanoseconds. These should be implemented as functions that can be used in annotation (with=ordeserialize_with=`.)

Custom implementation for custom types should reside with the type and not in the serializers library. (For example CommitSig.)

Together with putting everything in its place, the custom types that need deserialization should deserialize into a type that's easy to use for the developer. The Go types are not necessarily the best mirrored in Rust. Find the Rust-native implementations and make sure that custom serialization can make it compatible with the specification. (Try to separate the issue of serialization from the daily use of the type.)

[greg-szabo]

I'm breaking up PR #248 into multiple, easier-to-handle PRs.

[liamsi]

@greg-szabo I think all issues mentioned in the opening comment are addressed for now? And there is also a high-level serialization issue about the different kinds of serialization and overall structure #197. Feel free to re-open if I'm missing sth here.