Use std::equal<> as qfind default comparator

Use std::equal<> as qfind default comparator

Summary:
qfind(haystack, needle) is templated, but used a non-templated
default comparator that cast everything to char.  This works find for
8-bit types, but not for others such as the uint32_t Ranges we are
using.  Replace the default comparator with std::equal<> to use a
working comparator for whatever type is being searched.

Test Plan:
Compiles, unit tests (including a new one that fails with the
old code) pass.  Not sure how to test other users of this, or measure
performance impact. :(

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D906311