refactor(context): Rewrite and explain value merging logic

Changes the logic for merging context values to be unambiguous and
easy to follow.

* loadDefaultVars returns the default vars directly instead of
  performing merging in addition
* all merging is performed in `mergeContextValues` using explicit
  explanations for every step of the merge.

After this commit the order of merging goes from least to most
"specific", please read the explanatory comments for more information.

This relates to #142.

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