Redo primitive deserialization code to use no-param constructors

[ewoutkramer]

Currently, we need to have specific constructors for each primitive, but we can also use the no-param constructors and then set the ObjectValue. This will allow us to keep track of invalid values too.

[ewoutkramer]

I double checked the code in the SDK: all primitives have a Value property (that is no news) that casts an underlying ObjectValue property to the desired primitive type. This means our parsers and serializers can just read/set that ObjectValue instead of having to call the correct constructor.

[ewoutkramer]

Note that this will allow the parsers to parse 'illegal' values and store then in the FHIR primitive POCOs. The current "slow" POCO parser checks that the input is in the correct format before setting the property - but we might want to deal with that differently.