Remove mw.getLogBuffer and others from public interface

One of the design goals of Scribunto is that each #invoke should be
independent. Creative use of mw.log and mw.getLogBuffer can get around
this, passing information from one #invoke to the next.

This patch takes the simple solution of removing mw.getLogBuffer from
modules' environments. For good measure, it also removes
mw.clearLogBuffer and mw.executeModule.

Some minor cleanup of the console code is also included.

Change-Id: I30d73928bade4a6bdd7c00ffcd58a3858ff55698