Preserve array keys in PhutilReadableSerializer::printShallowRecursive()

Summary: See T582. When a user phlog()'s a value into DarkConsole and then
clicks it for details, printShallow() incorrectly rekeys arrays because PHP
subtly discards keys in array_slice(). Instead, explicitly preserve keys.

Test Plan:

• Added a phlog() for an array with keys 1, 2.
• Clicked it in DarkConsole for details, saw keys 0, 1.
• Applied patch, saw keys 1, 2.

Reviewers: Girish, tuomaspelkonen, edward

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 1029