Defines multimethod and implementations for step definitions.
A single step execution is the execute multimethod which dispatches
on the type of the first argument (step-descriptor).

If the step-descriptor is a map, the invocation will use :drtest.step/type key
as the value.

The first argument is the step descriptor. The second argument
is a context map which can carry any user defined values.
The third and fourth arguments are ok/fail callbacks.

If the step succeeds it will call ok callback with the new context map.
If the step fails it will call fail callback with the error and optional
error data map.