Implement type-level substitution of an arbitrary (open) session type for Done

[plaidfinch]

In order to support the development of the session! macro to fulfill #30, we need to implement a more general form of type-level substitution, namely substituting an arbitrary session type for Done to "extend" a session. This is needed because the ; operator in the macro language does not necessarily have the information to expand out a type synonym within the context of the macro invocation, since the type could be defined elsewhere, yet it needs to substitute the continuation (everything after the ;) for every Done in the preceding type.

This must be implemented for even open session types like Continue. For instance, imagine the following:

type P = session!(send i64);
type Q = session!(recv bool);
type S = session!(loop { P; Q });

In this example, we would want to compile to:

type P = Send<i64, Done>;
type Q = Recv<bool, Done>;
type S = Loop<Send<i64, Recv<bool, Continue>>>;

This requires us to substitute Continue for Done in Q (due to the fact that our macro language implicitly loops if not break-ed), and then Q for Done in P.

Note that the de Bruijn index associated with the Continue is not incremented as it is substituted, because the types of P and Q don't contain Loops, but in general an auxiliary function to increment de Bruijn indices in the substituted terms is necessary to make Continues point to the correct place.