Preserve string values on data set reading

[Enet4]

This resolves #9. It introduces configurable value reading strategies (ValueReadStrategy) at data set reading level and makes Preserve the default one. Quoting from the documentation:

Values will be stored without decoding dates or textual numbers.
Word-sized binary values are read according to the expected byte order.
Date-time values and numbers are kept in their original string representation as string objects.
All text is still decoded into Rust string values, in accordance to the standard, unless its value representation is unknown to the decoder.

With #56 already present, working with PrimitiveValue should be easy enough, performing the conversions to binary numbers and date/time/datetime only when necessary via the methods added there.

Note also that this does not exclude the eventual attribute abstraction, which will exist at the object level and even work with values which are not necessarily in memory. Everything should hopefully fit like puzzle pieces once this one is done.