Deserialize chart

[jasoncg]

Adds Deserialize support to various data structures in order to support deserializing Chart, as well as a couple extremely basic test cases to test deserialization.

Most of this was straightforward, just adding Deserialize to the derive attribute. But the following have custom Serialize implementations, so I wrote custom Deserialize implementations to match:

• datatype::Dataset
• series::Series
• series::ThemeRiverData
• element::axis_line
• element::BoundaryGap
• element::FontFamily
• element::Icon
• element::MarkLineVariant
• element::Padding
• element::RawString
• element::Symbol

The other change was some vectors have skip_serializing_if = "Vec::is_empty", which was causing problem with deserialization since there wasn't anything to initialize these vectors. For all of those I set #[serde(default)] so that they can deserialize with empty vectors.

[LukaOber]

A lot of effort went into this, thank you very much. Would you be able to create unit tests for each custom Deserializer?

[LukaOber]

I took a detailed look at the code. I tried implementing some unit tests myself and I think that the implemented Deserialization of Dataset is wrong, I haven't really tested the others, so likely more. Unit testing these implementations is definitely needed.

Do you have the time to add the unit tests? If not, I will try to correct the wrong Deserialization and provide some tests. But I am not super familiar with Deserialization.

I used serde_assert for my testing and I think it is a good choice for the unit tests. Just a simple roundtrip should be good enough.