Refactor `phutil_var_export`

Summary:
Using regular expressions in `phutil_var_export` to prettify the output is a pretty bad idea (albeit it is something that we have been using for a while). PHP's `var_export` function is pretty simple conceptually and we can probably just construct the "exported" variable representation ourselves in most cases (and just call out to `var_export` in other cases).

It's a little weird to make a prettiness/complexity tradeoff in favor of prettiness here, but I think humans end up interacting with the generated files often enough that it's somewhat valuable to have them follow conventions and look like other files.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9935