Generate slice getters/setters in the context

[sagikazarmark]

Is your feature request related to a problem? Please describe.
I'd like to manage slices in the context easily.

Describe the solution you'd like
Generate slice variants for the current set of primitive types.

Describe alternatives you've considered
Using plain Get is quite cumbersome at the moment. GetAs could be an alternative as proposed in #89

[bkielbasa]

personally, I'd prefer GetAs function. If we choose this solution we'll do the same for maps as well.