Align the values of ObjectValue with those of ITypedElement.

[ewoutkramer]

After having worked on the performance of FhirDateTime etc. I now realize that the ObjectValue for these types should be the System types DateTime, Date and Time. This means:

• The parsers will validate the format, and store the parsed system type in ObjectValue. Like with the other types the parser will put a string here if it fails to parse the datetime
• We will no longer have a random string in ObjectValue, so the constructor taking a string should be obsoleted and replaced with an explicit Parse() function.
• Value will change to be a DateTime/Time/Date as well.

This is obviously a breaking change, Is it too big?

[ewoutkramer]

In any case, the parser should validate this, otherwise this value will fall "between the cracks" of the parser and profile validator.