Calculate value of a type

[senier]

Sometimes the value of a type cannot be derived from scalar values or basic operations on scalar values, but are more complex calculation is required. Examples are variable length integers (#8) or the sum of the length fields of certain elements of an array (#215).

Create a concept for types to have values. Idea: Add a Value attribute to a type with an expression stating how to calculate the resulting value. The value can then be used in a specification through a 'Value attribute applied to the respective field.

Questions:

• What's the result type of the Value attribute
• Are there situations where we need multiple values for one type?
• How can a verifiable parser be derived from that?
• Do we need to restrict the allowable expressions to guarantee a verifiable parser?