Combine Builtins and Prelude

[brprice]

For historical reasons we have three places where we pre-define some types and/or terms: Builtins, Prelude and Primitives. The Primitives are special, since they are special support for non-algebraic types (Integer etc), so they should be left separate. However, Builtins and Prelude are both entirely user-definable, and their only reason for existence is to make the testsuite easier and to seed new programs. They should probably be combined (i.e. merge builtins into prelude).

(Somewhat related: #344 (comment))