Prep for GameLog stuff: Introduce ContextEnhancer

[Stefan-Hanke]

This PR is written as prep work for #227, where we want to attach a log object to the context, allowing a game to write custom logging information. It introduces a class called ContextEnhancer that ties all APIs together that can be attached and detached from a context - currently, these are random and events. This results in

• less code duplication, and
• easier attaching of new APIs.

Albeit I failed to also fixup the code creating the initial state, I've not exactly pinpointed why using ctxApi.update() does not work. The flow of the various state and context objects gets unwieldy, too. And perhaps there are other places that could use that class also.

Let me know what you think.

Checklist

• Use a separate branch in your local repo (not master).
• Test coverage is 100% (or you have a story for why it's ok).

[coveralls]

Coverage remained the same at 100.0% when pulling 7f31d59 on Stefan-Hanke:allow_payload_in_move into 3999db3 on google:master.

[nicolodavis]

This can be written more concisely as:

const ctx = { ...state.ctx, _random: this.state };
return { ...state, ctx };

[nicolodavis]

  state = apiCtx.update(state);
  initial.ctx = state.ctx;
  initial.ctx = apiCtx.detachFromContext(initial.ctx);

should work here.

A more interesting observation is given that update is always followed by a detachFromContext, shall we combine them into one call detach that accepts state and returns state?