Heterogeneous lookups for sorted_vector types

Heterogeneous lookups for sorted_vector types

Summary:
[Folly] Heterogeneous lookups for `sorted_vector` types.

When the `Compare` type has member type or alias `is_transparent`, enable template overloads of `count`, `find`, `lower_bound`, `upper_bound`, and `equal_range` on both `sorted_vector_set` and `sorted_vector_map`.

This is the protocol found in the equivalent `std::set` and `std::map` member functions.

> This overload only participates in overload resolution if the qualified-id `Compare::is_transparent` is valid and denotes a type. They allow calling this function without constructing an instance of `Key`.
>
> http://en.cppreference.com/w/cpp/container/set/count (same wording in all 10 cases)

Reviewed By: nbronson

Differential Revision: D6256989

fbshipit-source-id: a40a181453a019564e8f7674e1e07e241d5ab068