Prepend _ to value, discriminator in union

The use of members named "value" and "discriminator" in a union are problematic because of
the existence of properties on the IdlUnion class with these names.  (The error message are
quite confusing.)  For example, this:

    union U switch (boolean) {
        case TRUE: string value;
    };

would result in a union where you could not set the value.  You might be able to get it,
more or less by accident.  If instead of "value" the member would be named
"discriminator", it'd be even worse.

The IDL spec has a mechanism for dealing with these problems: the use of a leading
underscore as an keyword escape mechanism, both on the IDL and the language binding.  The
Python already used this for keywords, now it also uses it for these two identifiers in
unions.  Thus, the above now maps to:

    class U(idl.IdlUnion, discriminator=bool, discriminator_is_key=False, typename="U"):
        _value: types.case[[True], str]
        annotate.member_name("_value","value")

Trying to define a union with these members (without the leading underscore) now raises an
exception.  The underscore is not in the TypeObjects and also stripped in the IDL output
by `cyclonedds typeof`.

Signed-off-by: Erik Boasson <eb@ilities.com>