bytevec 0.2.0

• Rename the bytevec_impls macro to bytevec_decl. This macro now accepts a pub
  access modifier on the declared struct and on its fields.
• Reintroduce the bytevec_impls macro. This macro now can do a partial or full implementation
  of the serialization and deserialization operations for an externally declared struct. Partial
  implementation in this context meaning that it can be implemented for only a subset of the
  actual fields of the struct rather than for all the fields. The remaining fields will be
  obtained from the value returned from [Default::default()] on deserialization.
• Change the fixed u32 type for the size indicators used in the byte representation of a complex
  structure. The methods of ByteEncodable and ByteDecodable now use an integral type parameter
  Size constrained by the BVSize trait. This trait is implemented for u8, u16, u32 and u64.
  This now lifts the 4GB fixed limit of byte buffer, as the user now can use u64 for the size indicator.
• Add a decode_max method to the ByteDecodable trait, so users are now able to set a limit to the
  length of byte buffer on deserialization. If the limit is less or equal than the buffer length, it will
  call and return the value returned from decode, otherwise it will return a BadSizeDecodeError.
• Change endianness from big endian to little endian.