case senstivity refactoring: use spatch instead of Perl

Summary:
I was using perl and got some false positives while fixing
the case of some identifiers. Safer to use spatch.

Test Plan:
unit test included
./spatch -test

Reviewers: jiaol, jyan, pieter, julienv, amenghra

Reviewed By: jyan

Differential Revision: https://phabricator.fb.com/D873287
* main_spatch.ml: more stuff