Support for invoking futures (CLJ only) from statecharts. This support can be added by
adding it to the env's `::sc/invocation-processors`. An invoke element can specify it wants to
run a function in a future specifying the type as :future.

The `src` attribute of the `invoke` element must be a lambda that takes one argument (the
params of the invocation) and returns a map, which will be sent back to the invoking machine
as the data of a `:done.invoke.invokeid` event.

If the invocation is cancelled (the parent state is left), then future-cancel will be called on the future.

Support for invoking other statecharts. This support can be added by adding it to the
env's `::sc/invocation-processors`. An invoke element can specify it wants to
run another statechart by specifying the type as :statechart, ::sc/chart, or the official w3 URL for scxml.

The `src` attribute of the `invoke` element must be the name of a machine that is in the
statechart registry (in env as ::sc/statechart-registry).