Access pattern

[jemc]

We want to build a convention (and maybe even some form of additional language sugar) around the Access Pattern.

The TL;DR is that we want to be able to pass a lambda to be executed in the context of a given actor, with the actor passing itself in as the argument of that lambda.

More thought is needed into how we want to represent this in Savi in the most convenient/clean way, but at its most basic we could just use a normal lambda, as in Pony: #130

[jemc]

This is blocked until we have objectifiable lambdas, as proposed in #130

[mneumann]

Proper API of Postgres library depends on this, too. For now I can use a Notify.