Make parameter classes less generic and add factory methods

There are now only three specific parameter classes: RequiredSingle,
OptionalSingle, and Multiple, corresponding to the three possible value
types (String, Optional<String>, and List<String>). Parameter objects
are no longer created directly via constructors, but using factory
methods. This is shorter and decouples the parameter creation from
their implementation.

There is also a new Parameter.body factory method to create body
parameters, which previously used the same class/constructor as
required remainder parameters.

The PositionalParameter namespace class is now completely removed; all
parameter-related classes and methods are now under the Parameter
class.