Clean up more code (#30)

* Rename scopeKey() to getScopeKey()

* Move auto-creating getScopeInfo to getOrCreateScopeInfo

* Use null coalesce to shorten getScopeInfo

* Move auto-creating getVariableInfo to getOrCreateVariableInfo

* Remove redundant comment

* Remove useless return value in markVariableReadAndWarnIfUndefined

* Remove assignments inside conditions

* Remove unnecessary comments

* Extract the loop in processScopeClose to its own function

* Remove old third arg to getVariableInfo

* Remove null coalesce

Author: sirbrillig