src/template/base.php: In 'renderFromString' and 'renderFile', delete (or, 'unlink') temporary file after rendering the underlying class, but before returning the result.
src/template/base.php: Some refactoring/tidying-up; wrap similar/same code used by both 'renderFromString' and 'renderFile' into 'renderExpandedTemplateAndUnlinkTmpFile'.
src/template/base.php: Rename 'renderFromString' to just 'renderString', leaving a @deprecated wrapper to it as the old name; update test cases as appropriate.