sgrep: start of unit tests for the generalized sgrep

Test Plan: ./sgrep -test

Reviewers: pieter

Reviewed By: pieter

Differential Revision: https://phabricator.fb.com/D799566
* matcher/fuzzy_vs_fuzzy.ml: misc
* matcher/unit_matcher.ml: misc unit tests
* matcher/lib_unparser.ml: comments
* matcher/sgrep_fuzzy.ml: more comments
spatch: start of unit tests for the generalized spatch

Test Plan: ./spatch -test

Reviewers: pieter

Reviewed By: pieter

CC: ptarjan

Differential Revision: https://phabricator.fb.com/D799778