Positional factory function for class io.pedestal.http.servlet.FnServlet.

Returns an instance of javax.servlet.Servlet using provided
functions for its implementation. Arguments are key-value pairs of:

  :init      optional, initialization function taking two arguments:
             the Servlet and its ServletConfig

  :service   required, handler function taking three arguments: the
             Servlet, ServletRequest, and ServletResponse

  :destroy   optional, shutdown function taking one argument: the
             Servlet

The :init, :service, and :destroy options correspond to the Servlet
interface methods of the same names.

Note: this function returns an instance, not a class. If you need a
class with a static name (for example, to deploy to a Servlet
container) use the Java class pedestal.servlet.ClojureVarServlet.