Use phutil_var_export for PhutilParserGenerator::formatAndIndent

Summary: phutil_var_export produces more readable output than var_export. Depends on D9923.

Test Plan: Eye-balled the PhutilTypeSpec code.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9925