Add support for record and multiendian

[jlukas]

record can be used to specify dynamic types as fields of a record or struct:

def type(arguments)
  BinData::Struct(...)
end

class A < BinData::Record
  record :field, type: type(arguments)
end

multiendian can be used to automatically duplicate a type for big and little endian

BinData::Record.multiendian(:type_name) do
  uint32 :field
end

class A < BinData::Record
  endian :little
  type_name :field # knows to use little-endian version of type_name
end

class B < BinData::Record
  endian :big
  type_name :field # knows to use big-endian version of type_name
end

[dmendel]

Thanks for your code proposal.

I've committed a change to master so that the endian keyword applies to custom types as well as built in numerics.

class CoordLe < BinData::Record
  endian :little
  int16  :x
  int16  :y
end

class CoordBe < BinData::Record
  endian :big
  int16  :x
  int16  :y
end

class Rectangle < BinData::Record
  endian :little
  coord  :upper_left     # <-- Here CoordLe is automatically
  coord  :lower_right    # <-- assumed
end

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Your record keyword:

I think the change I previously noted removes the need for this keyword. Let me know if you have a use case where record is needed.

A declaration shouldn't contain implementation details such as whether a type is dynamically defined. This keyword makes declarations over complicated.

---

I'm still considering the multi endian declaration. If I implement it, it will have to apply to Records, Primitives, Arrays and Choices. At the moment, I don't like the proposed syntax as it is too different to current declarations.