GVariant: use GVariantTypeInfo on internal constructors

GVariant: use GVariantTypeInfo on internal constructors

Take GVariantTypeInfo instead of a GVariantType on the internal
constructors for GVariant.

Although this results in a bit more code for almost every call, it turns
out that it makes most uses more convenient -- that fact that we consume
the GVariantInfo means that we don't have to go back to free the type
when we're done.

This change will allow us to port g_variant_get_child() over to using
g_variant_alloc().  It was the one outstanding place where we
constructed GVariant instances that was not yet using it.