Adds hook scope aliases `example` and `context`
Automatically generates scope/alias names in error messages
Reworks example-group scope-alias specs to be less ambiguous
Refactors scope_aliases method ⇒ SCOPE_ALIASES constant hash
Lifts new private instance methods to singleton methods
Merge pull request #1174 from fj/topic/scope-aliases

Adds hook scope aliases `example` and `context`