Support attributes and direct element content in Sassone.Builder/Sassone.XML.build

[lucacorti]

It is desirable to be able to honor attribute direct element content mapping in Sassone.Builder in XML.

e.g. when dealing with this legal XML representation:

<person gender="male">
  <name>Bob</name>
  <surname>Price</surname>
  Some other content.
</person>

Current status

• Serialize attributes defined via @derive Sassone.Builder
• Serialize direct element content via @derive Sassone.Builder (e.g. "Some other content." above)
• Parse attributes defined via @derive Sassone.Builder to struct field (e.g. person element gender attribute above)
• Parse direct element content via @derive Sassone.Builder to struct field (e.g. "Some other content". above)