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