Improve the performance of phutil_loggable_string()

Summary:
Ref T4162.

• Before doing anything character oriented, test if the whole string is fine to print as-is.
• Reduce the number of calls to string().

Test Plan: Saw cost drop on profiles.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4162

Differential Revision: https://secure.phabricator.com/D7656