Add structure default constructors

[emgre]

• Fix Default values for structure fields #20 and fix Struct constructor/builder pattern #33.
  • It is now possible to define a default value to any struct field.
  • If all the fields of a struct have a default value, then it automatically has a default value if used within another struct
  • In C# and Java, structs now have a constructor that takes all the values that do not have a default value.
  • All documentation states the default value (if there is one)
  • In C, static (from a translation unit perspective) functions named {struct_name}_init are provided. They work the same way as the constructors in C# and Java.
• Fix Provide Action<T> for interfaces with a single callback #34
  • In C#, when an interface or a one-time callback is functional (it only contains a single callback), then a implementation of the interface is generated. Its constructor takes an Action or a Func which can be a lambda function.
  • In Java, the FunctionalInterface is added to these interfaces.