Upgrade to Discount 3.x

[davidfstr]

Motivation:

• Discount 2.x does not build unmodified on newer compilers / operating systems.

Considerations:

• Breaking changes to the RDiscount API are on the table, as permitted socially by a major version update
  • Note some proposed API simplifications via sunaku
  • Still, avoid large breaking changes where possible. I don't want to have a Python 2.x -> 3.x experience.