Support for instanceof

[jeromesimeon]

It would be great to be able to match values against types. This could be a new operator in the language e.g., e instanceof T or e is of type T.

This could also be used for the underlying semantics of define variable x : T := e.

I believe instanceof can be supported as an operator and compiled to an underlying combination of data-type inference and sub typing (See querycert/qcert#95).

Additionally, this might allow us to consolidate the pattern matching expression, which currently only works for either class names or values, and from there to generalize the type dispatch implemented for #15