Function types

[cdmistman]

These can be implemented internally to enable things like fn map, etc. This will be necessary to fix fn selection in the compiler. While exposing complex function types to users (eg Function<Arg0, void> as proposed in RFC 11) is blocked on #537, a workaround with the current Alan function type is possible by exposing a ducked Type in Types.ts that accepts any new internally-recognized Function type (or whatever the class is defined in Types.ts is). This will enable the PR for #537 to expose the new Alan Function<P, R> type